Invertebrate Anatomy OnLine
Pollicipes polymerus ©
with notes on Lepas anatifera
Copyright 2001 by
This is one of many exercises available from Invertebrate Anatomy OnLine , an Internet laboratory manual for courses in Invertebrate Zoology. Additional exercises can be accessed by clicking on the links to the left. A glossary and chapters on supplies and laboratory techniques are also available. Terminology and phylogeny used in these exercises correspond to usage in the Invertebrate Zoology textbook by Ruppert, Fox, and Barnes (2004). Hyphenated figure callouts refer to figures in the textbook. Callouts that are not hyphenated refer to figures embedded in the exercise. The glossary includes terms from this textbook as well as the laboratory exercises.
Arthropoda P, Mandibulata, Crustacea sP, Eucrustacea, Maxillopoda SC, Progonomorpha, Thecostracomorpha, Thecostraca, Cirripedia C, Thoracica O, Lepadomorpha sO, Scalpellidae F, (Fig 16-15, 19-89, 19-90)
Arthropoda, by far the largest and most diverse animal taxon, includes chelicerates, insects, myriapods, and crustaceans as well as many extinct taxa such as Trilobitomorpha. The segmented body primitively bears a pair of jointed appendages on each segment. The epidermis secretes a complex cuticular exoskeleton which must be molted to permit increase in size. Extant arthropods exhibit regional specialization in the structure and function of segments and appendages but the ancestor probably had similar appendages on all segments. The body is typically divided into a head and trunk, of which the trunk is often further divided into thorax and abdomen.
The gut consists of foregut, midgut, and hindgut and extends the length of the body from anterior mouth to posterior anus. Foregut and hindgut are epidermal invaginations, being derived from the embryonic stomodeum and proctodeum respectively, and are lined by cuticle, as are all epidermal surfaces of arthropods. The midgut is endodermal and is responsible for most enzyme secretion, hydrolysis, and absorption.
The coelom is reduced to small spaces associated with the gonads and kidney. The functional body cavity is a spacious hemocoel divided by a horizontal diaphragm into a dorsal pericardial sinus and a much larger perivisceral sinus. Sometimes there is a small ventral perineural sinus surrounding the ventral nerve cord.
The hemal system includes a dorsal, contractile, tubular, ostiate heart that pumps blood to the hemocoel. Excretory organs vary with taxon and include Malpighian tubules, saccate nephridia, and nephrocytes. Respiratory organs also vary with taxon and include many types of gills, book lungs, and tracheae.
The nervous system consists of a dorsal, anterior brain of two or three pairs of ganglia, circumenteric connectives, and a paired ventral nerve cord with segmental ganglia and segmental peripheral nerves. Various degrees of condensation and cephalization are found in different taxa.
Development is derived with centrolecithal eggs and superficial cleavage. There is frequently a larva although development is direct in many. Juveniles pass through a series of instars separated by molts until reaching the adult size and reproductive condition. At this time molting and growth may cease or continue, depending on taxon.
Mandibulata is the sister taxon of Chelicerata and in contrast has antennae on the first head segment, mandibles on the third, and maxillae on the fourth. The brain is a syncerebrum with three pairs of ganglia rather than the two of chelicerates. The ancestral mandibulate probably had biramous appendages and a J-shaped gut, posterior-facing mouth, and a ventral food groove. The two highest level mandibulate taxa are Crustacea and Tracheata.
Crustacea is the sister taxon of Tracheata and is different in having antennae on the second head segment resulting in a total of 2 pairs, which is unique. The original crustacean appendages were biramous but uniramous limbs are common in derived taxa. The original tagmata were head but this has been replaced by head, thorax, and abdomen or cephalothorax and abdomen in many taxa. Excretion is via one, sometimes two, pairs of saccate nephridia and respiration is accomplished by a wide variety of gills, sometimes by the body surface. The nauplius is the earliest hatching stage and the naupliar eye consists of three or four median ocelli.
Eucrustacea includes all Recent crustaceans except the remipedes. The taxon is characterized by a primary tagmosis consisting of heat, thorax, and abdomen although the derived condition of cephalothorax and abdomen is more common. Eight is the maximum number of thoracic segments.
Maxillopodans are short crustaceans with 10 of fewer trunk segments including a maximum of seven thoracic segments. The abdomen consists of three segments, none of which have appendages. The naupliar eye has three, instead of four, pigment cups. Maxillopoda includes such well known taxa as copepods, barnacles, ostracods, and branchiurans as well as more obscure groups like tantulocaridans and pentastomidans.
Barnacles are a diverse taxon including free-living and parasitic groups. Barnacles are sessile and most are hermaphroditic. Most are suspension feeders but many are parasites and some are predators. The exoskeleton usually secretes heavy calcareous plates which grow with the animal and are not molted, although the rest of the exoskeleton is.
Thoracic barnacles are typically sessile, non-parasitic, suspension feeders although some are also predators. Most live attached to hard substrata in coastal waters where they use their setose thoracic appendages to filter particulate food or capture prey from the water. The dorsolateral body wall is evaginated and folded to form a bilobed carapace similar to the mantle of bivalve molluscs and brachiopods, and accordingly known as the mantle. The mantle encloses a mantle cavity and the body and also secretes a calcareous, multivalved shell.
The body consists of a head, thorax, and vestigial abdomen. There is no cephalothorax and no maxillipeds although the carapace (mantle) encloses the entire body including the complete thorax. The head is divided into a preoral region anterior to the mouth and a postoral region behind it and bears little resemblance to the heads of other crustaceans. The preoral region is much enlarged and elaborated to form an attachment organ by which the animal adheres permanently to its substratum. The rest of the body is postoral and consists of the remainder of the head, the thorax, and a tiny abdomen. In lepadomorph, or stalked, barnacles, the preoral region of the head is a flexible, muscular stalk. The postoral region is enclosed in the mantle, mantle cavity, and shell. The preoral head of balanomorph, or acorn, barnacles does not form a stalk and instead attaches directly to the substratum.
The lepadomorph barnacle, Pollicipes polymerus (= Mitella polymerus), occurs in large numbers attached to intertidal rocks on the North American west coast of from Mexico to British Columbia. It is readily available for use at institutions near rocky coasts bordering the eastern Pacific and is also available alive or preserved from supply houses. A pelagic lepadomorph species, Lepas anatifera (Lepadidae), occurs attached to floating objects at sea but is occasionally found washed ashore on the Pacific and Atlantic coasts of North America, and elsewhere. It is also available, alive and preserved, from supply houses. This exercise is written for Pollicipes but can be used for Lepas as well. The major differences between the two lie in the shell plates. Parenthetical comments point out pertinent differences in the two.
The dissection should be conducted in a small dissecting pan on the stage of a dissecting microscope. The dissection may be performed on living or preserved specimens. Living specimens should be in isotonic magnesium chloride. About an hour is required for specimens to succumb to the relaxant. Preserved animals should be immersed in tapwater.
Examine the external features of your specimen with the dissecting microscope.
The body of Pollicipes, and other stalked barnacles, consists of a flexible, tough stalk, the peduncle, and a capitulum at the free end of the stalk (Fig 1, 19-80A).
The peduncle is chiefly an attachment organ but also contains the ovaries and some muscles. It is a short fleshy stalk covered with tiny calcareous ossicles. (The peduncle of Lepas has no ossicles.) The peduncle is the dorsal part of the animal and contains the preoral region of the head. The barnacle attaches to the substratum by the peduncle and first antennae. Barnacles have no second antennae.
The capitulum, on the other hand, consists of most of the animal, including all appendages (except for the first antennae), the gut, mantle (carapace), and mantle cavity. It is covered by a series of large calcareous plates and numerous smaller ones. (The small plates are absent in Lepas.) The capitulum is ventral and is held away from the substratum by the peduncle.
Figure 1. The left side of the barnacle, Pollicipes polymerus. Barnacle13L.gif
The aperture is a longitudinal slit along the free edge of the capitulum through which the animal communicates with the surrounding sea. The aperture is the opening into the mantle cavity and is on the ventral surface of the animal.
Look at Pollicipes from the side and note that one end of the ventral edge of the capitulum is higher than the other (Fig 1). The higher end is posterior and the opposite, lower end is anterior. Find the right and left sides. (In Lepas (Fig 2) the posterior end is that with the single, unpaired calcareous plate, or carina.)
Figure 2. The left side of the barnacle, Lepas anatifera. Note the absence of the rostrum in Lepas. Barnacle14L.gif
The barnacle exoskeleton is thin and flexible, being composed of unsclerotized protein and chitin. In addition, the exposed parts of the body are covered by numerous calcareous plates secreted by the epidermis (Fig 1, 19-80A).
The capitulum is covered with plates of various sizes. The largest of these are ventral, on the free edge of the capitulum around the aperture. In Pollicipes, the plates decrease in size approaching the peduncle and the smallest are on the peduncle itself. The peduncle bears hundreds of tiny scalelike plates, each sitting atop a short stalk. (The peduncle of Lepas is naked and bears no scales.)
The standard six plates typical of most thoracic barnacles are present on the capitulum of Pollicipes. (Lepas is missing one of the six plates.) The two large plates beside the posterior end of the aperture are the terga (= back). The two large plates beside the anterior end of the aperture are the scuta (= shield). The four scuta and terga form the two sides of an operculum which moves laterally to open the aperture for feeding and respiration. The rostrum and carina are unpaired plates lying at the anterior (rostrum) and posterior (carina) ends of the aperture respectively. (Lepas has no rostrum.)
Pollicipes (but not Lepas) has many additional plates, known as accessory plates, some of which have individual names, around the dorsal half of the capitulum. The first row of accessory plates are the laterals (Fig 1). These are the largest of the accessory plates. The one closest to the carina is the carinolateral, whereas that closest to the rostrum is the rostrolateral. These laterals are homologous to the immovable side walls of acorn barnacles such as Balanus (Fig 19-78B).
(Lepas has neither accessory plates nor rostrum (Fig 2, 19-80A). The apex of the capitulum of Lepas is the two terga. The very large plates on either side are the scuta. The long, narrow, unpaired carina is on the posterior edge.)
Pull the sides of the aperture apart and look inside at the space within. This is the mantle cavity (Fig 19-78A). The cavity is enclosed by the mantle, which is a double fold of the body wall homologous to the carapace of other crustaceans. The postoral region of the body (thorax and most of the head) lies in the mantle cavity. The shell plates are embedded in the outer surface of the mantle. The inner surface of the mantle is dark and uncalcified. At present you can see little more than the tentacle-like feeding appendages in the mantle cavity.
Place the barnacle on its left side in the dissecting pan. The right side is up, the peduncle is toward you, and the aperture away from you. The barnacle's posterior end is to your right.
" Locate the irregular line separating the dorsal edge of the lateral plates from the ventral edge of the next row of small (unnamed) accessory plates. At the posterior end, carefully force the fine point of a pair of small but strong and sharp scissors into a gap between two plates on the line mentioned above. Push the blade through the mantle, but no deeper, and cut between adjacent plates along this line. Do not cut the plates themselves. It is not necessary and will ruin your scissors. Cut anteriorly until you reach the rostrolateral plate then cut between it and the rostrum. Continue posteriorly along the line until you reach the carinolateral and pass between it and the carina.
Carefully lift the right mantle lobe and look beneath it to locate any tissues still holding it in place. The large scutal adductor muscle runs transversely from one scutum to the other and must be cut.
The right mantle, with its plates, is now entirely free and should be removed. The cavity exposed by your efforts is the mantle cavity and in it can be seen the postoral head and thorax.
" (For Lepas, use fine scissors to cut around the scutum and tergum of the right side. Pull the two scuta apart gently and cut the large scutal adductor muscle which holds them together. Remove the right scutum and tergum, leaving the carina and left scutum and tergum intact.)
For Pollicipes and Lepas, force one or two # 1 stainless steel insect pins through the peduncle to hold the animal in place while you study it.
Notice the brown mantle covering the inside of the remaining shell plates. Find the postoral body consisting of the thorax and part of the head (Fig 3, 19-78A). It is suspended in the mantle cavity and attached only by the head to the mantle and peduncle. The animal is upside down in the mantle cavity with its ventral surface pointed toward the aperture and its dorsum toward the peduncle. Find the head at the anterior end of the body. It is the only part attached to the outer mantle and peduncle and it suspends the animal in the mantle cavity (Fig 3, 19-78A).
Figure 3. View of the right side of Pollicipes with the right mantle lobe and right thoracopods removed. The cirri are numbered. Branacle15L.gif
The scutal adductor muscle lies immediately ventral to the attachment of the head to the mantle (Fig 3). Look posterior to the muscle for the postoral region of the head and its appendages.
The first antennae are at the base of the peduncle and cannot be seen. The anterior part of the capitulum and the peduncle are the greatly enlarged preoral region of the head. Barnacles have no second antennae.
The characteristic three pairs of crustacean mouthparts are present. Immediately posterior to the scutal adductor muscle, on the ventral midline of the head, is a large protuberance known as the mouth cone composed of the mouthparts and other structures surrounding the mouth.
The anterior surface of the mouth cone is the labrum, which is an unpaired fold of body wall, not a segmental appendage (Fig 4). On either side of the labrum is a long, fingerlike process called the palpus (Fig 4). (In Lepas there is no palpus). Like the labrum, it is not a segmental appendage.
The mouth is immediately posterior to the labrum, but is hidden from view by the mouthparts and is hard to see. Twist the tip of the mouth cone toward you so you can see the mouth.
Figure 4. Mouthparts of Pollicipes, viewed from the right. Barnacle16L.gif
Moving posteriorly, the first pair of appendages are the large mandibles (Fig 4). Each has a formidably toothed medial margin and a large setose mandibular palp. The mandibles are immediately posterior to the labrum and beside the mouth. Next are the small first maxillae, which also have toothed medial margins. (The first maxillae of Lepas are setose but are not toothed.) The posteriormost mouthparts, the second maxillae, are immediately lateral to the ventral midline and posterior to the first maxillae. Each has a dense array of setae on its free distal end.
The body posterior to the mouth cone is the thorax. It bears six similar pairs of large biramous thoracopods known as cirri, or (Fig 3). The two rami of each thoracopod are elongate, curled, and whiplike (cirrus = curl, so the name “Cirripedia” means “curl foot”). The cirri bear abundant and conspicuous long setae. The cirri are the feeding appendages and are extended through the aperture like a cast net to capture food (Fig 19-80B). Find the six pairs of cirri. (Lepas has a pair of filamentous processes at the base of the first cirri.)
Although barnacles are usually described as suspension feeders, Pollicipes is a predator that uses its cirri to capture small crustaceans and other zooplankters. (Lepas also catches and eats macroscopic animals.) Most barnacles, however, feed on microscopic phytoplankton, captured by the fine mesh of setae on the cirri.
Pollicipes attaches to large rocks in the intertidal. It does this in such a way that the cirri face away from the sea in order to filter the water of crashing waves as it returns to the sea.
Barnacles are hermaphroditic. The very long, highly extensible penis arises on the ventral midline between the bases of the last, or sixth, pair of cirri (Fig 3). The male gonopore is at the tip of the penis. The penis can be extended from the aperture to deposit sperm in the mantle cavity of nearby barnacles (Fig 19-82A). The two female gonopores are at the base of the first cirri, one on each side of the anterior thorax.
The anus is on the midline between the bases of the sixth cirri, posterior to the base of the penis. In Pollicipes, but not Lepas, there is a fringe of conspicuous, fingerlike filamentary appendages around the dorsal margin of the head (Fig 3). They contain some of the follicles of the testes.
When mating, one partner functions as female and the other as male. Fertilization is external and occurs in the mantle cavity of the individual acting as the female. The male partner deposits sperm in her mantle cavity to fertilize eggs released from the female gonopores. The eggs are brooded in the mantle cavity. Two thin membranous ovisacs enclose the eggs in the mantle cavity during brooding and prevent the loss of eggs into the feeding current. When present, the ovisacs, filled with eggs or larvae, are large and conspicuous. The ovisacs are produced by the ovisac gland whose location is revealed by the bulge at the base of each first cirrus (Fig 3). It is an expansion of the distal end of the oviduct. Development produces a nauplius larva which is the hatching stage (Fig 6, 19-79A). Barnacle naupli are characterized by their distinctive secretory frontal horns. Like other naupli they hatch with three pairs of appendages; the first and second antennae and the mandibles, and have a naupliar eye. Subsequent molts transform the nauplius into a cypris with a bivalved carapace. The cypris resembles an ostracod and is the characteristic bivalved barnacle larva (Fig 19-79B).
>1a. If your specimen has ovisacs, remove one and make a wetmount of a small piece of it. Examine it with the compound microscope. Inside you will see eggs or larvae. <
" Use your fine scissors to make a longitudinal incision through the body wall along the entire length of the right side of the postoral body to open the hemocoel. Cut from head to the posterior end of the body. The incision should lie ventral to the filamentary appendages and dorsal to the bases of the cirri (Don't forget, the animal is upside down.) (In Lepas the incision should be just dorsal to the bases of the cirri.) Do not cut into any of the organs contained within the body. A large bundle of muscles extends from the rostrum into the dorsal side of the anterior end of the body. The organ systems are discussed in the order in which they are encountered.
Barnacles are hermaphroditic and perhaps the most noticeable feature within the body of living specimens is the bright white seminal vesicle of the male system (Fig 5, 19-78A). In preserved animals it looses its color and may be inconspicuous. It is a thick, twisting tube running longitudinally along the length of the thorax. A much smaller, also white, vas deferens lies along the ventral border of the seminal vesicle and joins it anteriorly.
The vas deferens drains the abundant and widely scattered testicular follicles to the seminal vesicle. The follicles are tiny, inconspicuous, more or less spherical bodies scattered throughout the interior of the body and the filamentary appendages of Pollicipes.
The seminal vesicle extends posteriorly along the bases of the thoracic appendages to the base of the penis. Here it joins the seminal vesicle from the other side to form a common sperm duct (Fig 5) which extends the length of the penis to the male gonopore.
Figure 5. Internal anatomy of Pollicipes viewed from the right side. Barnacle17L.gif
Sperm produced in the testes travel via the vasa deferentia to the seminal vesicles where they are stored. During reproduction it is delivered to the penis by the sperm duct.
>1b. If your specimen is living, remove a piece of the seminal vesicle to a drop of seawater on a slide. Tease the tissue apart to disperse the sperm, affix a coverslip and examine with high power of the compound microscope. Find sperm and note that they are flagellated. How does this compare with the sperm of most arthropods? <
The two ovaries are located in the peduncle and connect to the female gonopore by a pair of inconspicuous oviducts. You will see the ovaries later when you open the peduncle.
Relocate the mouth at the tip of the mouth cone (Fig 5, 19-78A). It opens into a long foregut derived from the embryonic stomodeum and hence is ectodermal. As an ectodermal derivative it is lined with exoskeleton.
" Slip a fine scissors into the mouth and cut dorsally on the right side of the head to open the foregut. Note heavy chitinous lining and widely spaced longitudinal ridges and grooves of the foregut (Fig 5).
At the end of the foregut there is a stomodeal valve marking the beginning of the midgut. The midgut is endodermal and consequently is not lined with exoskeleton. The midgut is longitudinally ridged also. The anterior region of the midgut is sometimes called the stomach (Fig 5, 19-78A). It is surrounded by the digestive ceca.
Figure 6. A barnacle nauplius larva from Beaufort, North Carolina. Barnacle21L.gif
Upon reaching the dorsal side of the body, the midgut curves posteriorly and follows the curve of the dorsal edge of the body to the posterior end of the thorax. In this region its diameter gradually diminishes and it is known as the intestine.
At about the level of cirrus 6 the midgut becomes the hindgut, or rectum. This short region extends to the anus, located at the posterior base of the penis between the two sixth cirri. The hindgut is ectodermally derived and is lined with exoskeleton.
>1c. Remove the contents of the midgut and make a wetmount. Observe it with the compound microscope and see if you can confirm the claim that Pollicipes feeds on crustaceans. Do you see any recognizable animals or parts of animals? Look for pieces of exoskeleton with setae as evidence of a crustacean diet. <
" Make a longitudinal incision, with scissors, through the integument of the peduncle extending from the base of the capitulum to the base of the peduncle.
Deflect and pin aside the cut integument and look inside. Find the layer of circular muscle just inside the thick integument. Inside the circular muscle is a well-developed layer of longitudinal muscle. Deepen the original incision, if necessary, so it cuts through these two layers. In the core of the peduncle are the ovaries. Each is composed of numerous, curled, yellow (in life) tubules which ultimately connect to an oviduct extending to the female gonopore. You may not find the oviduct.
Hemal, Excretory, and Nervous Systems
Study of these systems is not practical in these small animals.
Abbott DP . 1987. Observing Marine Invertebrates. Stanford Univ. Press, Stanford. 380p.
Ho J-S . 1978. Laboratory Manual for Invertebrate Zoology Emphasizing Marine Forms. Hwong, Los Alamitos, Calif. 152p.
Lochhead JH . 1950. Lepas anatifera, pp. 413-418, in Brown FA. (ed). Selected Invertebrate Types. Wiley, New York. 597p.
Ruppert EE, Fox RS, Barnes RB. 2004. Invertebrate Zoology, A functional evolutionary approach, 7 th ed. Brooks Cole Thomson, Belmont CA. 963 pp.
Walker G. 1992. Cirripedia. in Harrison, F. W. & A. G. Humes (eds.). Microscopic Anatomy of Invertebrates vol. 9 Crustacea . Wiley-Liss, New York. pp249-312.
Zullo VA. 1979. Marine flora and fauna of the northeastern United States. Arthropods: Cirripedia. NOAA Tech Rep. NMFS Circ 425: 1-28.
Small dissecting pan (sardine tin with wax bottom)
Living or preserved goose barnacle
Isotonic magnesium chloride for living specimens
# 1 stainless steel insect pins
Preserved Pollicipes and Lepas are available from Carolina Biological and Ward's Natural Science.
Lepas, Pollicipes, and Scalpellum are available from Pacific Bio-Marine.