Invertebrate Anatomy OnLine
Serpula vermicularis ©
Feather Duster Worm
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, a glossary, and chapters on supplies and laboratory techniques are also available at this site. 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.
Annelida P, Polychaeta C, Palpata, Canalipalpata, Sabellida O, Serpulidae F (Fig 13-7A)
Annelida consists of the segmented worms in the major taxa Polychaeta (bristleworms), Oligochaeta (earthworms and relatives), Branchiobdellida (crayfish ectosymbionts), and Hirudinea (leeches) with a total of about 12,000 known species in marine, freshwater, and terrestrial environments. The segmented body is composed of an anterior prostomium, a linear series of similar segments, and a posterior pygidium. The prostomium and pygidium are derived from anterior and posterior ends of the larva whereas the intervening segments arise through mitotic activity of mesodermal cells in the pygidium.
The body wall consists of a collagenous cuticle secreted by the monolayered epidermis. A connective tissue dermis lies beneath the epidermis. The coelom is lined by a peritoneum which may be specialized to form the body wall muscles. Most annelids have chitinous bristles, or chaetae, secreted by epidermal cells, that project from the body. The coelom is large, segmentally compartmented, lined by peritoneum, and well developed in polychaetes and oligochaetes but reduced in leeches. Successive coelomic spaces are separated by transverse bulkheads known as septa which consist of double layers of peritoneum with connective tissue in between. The right and left sides of each segmental coelom are separated by longitudinal mesenteries which, like septa, are double layers of peritoneum with connective tissue between.
The gut is a straight, regionally specialized tube that begins at the mouth at the anterior end and extends for the length of the body to end at the anus on the pygidium. It penetrates each septum and is supported by dorsal and ventral mesenteries. Like that of most invertebrates, the gut consists of ectodermal foregut, endodermal midgut, and ectodermal hindgut. The nervous system consists of a dorsal brain in or near the prostomium, a pair of circumpharyngeal connectives around the anterior gut, and a double, ventral nerve cord with paired segmental ganglia and nerves. The hemal system of most annelids is a set of tubular vessels, some of which are contractile and serve as hearts. The hemal system is absent or greatly reduced in leeches. The system includes a dorsal longitudinal vessel above the gut in which blood moves anteriorly, a ventral longitudinal vessel below the gut, in which blood moves posteriorly, and paired segmental vessels that connect the dorsal and ventral vessels. The digestive, hemal, and nervous systems are continuous and pass through the segments.
Respiration is accomplished in a variety of ways. In some, the general body surface is sufficient but gills are present in most polychaetes, many leeches, and a few oligochaetes. Excretory organs are metanephridia or protonephridia and typically one pair is present in each segment. These osmoregulatory organs are best developed in freshwater and terrestrial species. The sexes are separate in polychaetes but oligochaetes and leeches are hermaphroditic. In the ancestral condition paired submesothelial clusters of germ cells were present in each segment and released developing gametes into the coelom. In derived taxa reproductive functions tend to be confined to a few specialized genital segments. Gametes mature in the coelom or its derivatives and fertilization is external. Gametes are shed through ducts derived from metanephridia or by rupture of the body wall. Spiral cleavage follows fertilization. Clonal reproduction is common.
Polychaeta is a large (8000 species) and diverse taxon of marine annelids thought to be the most primitive of the annelid taxa and the most like the ancestral annelid. The body of a typical polychaete is divided into segments, each of which bears a pair of fleshy appendages, or parapodia. The head is often equipped with abundant, well-developed sense organs. The anterior gut is muscular, sometimes eversible, and frequently equipped with chitinous jaws. Polychaetes are gonochoric and gametes ripen in the coelom from which they are shed through ducts or by rupture of the body wall.
The prostomium has a pair of sensory palps which are lacking in the sister taxon, Scolicida.
The prostomium has palps with longitudinal grooves. Although derived from the sensory palps of ancestral polychaetes, these are adapted for either deposit or suspension feeding. Canalipalpatans were once known as “sedentary” polychaetes because they are sessile and live in tubes or burrows which they rarely, if ever, leave.
Sabellidans are suspension feeders with the mouth surrounded by a ring or feeding tentacles. The prostomium is fused with the peristomium. They inhabit permanent tubes which may be constructed of parchment, sediment particles, calcium carbonate, sand, or shell fragments. Sabellida includes, but is not limited to, the similar taxa Sabellidae, Serpulidae, and Spirorbidae.
Serpulid polychaetes are sedentary, suspension feeding worms that secrete and inhabit a calcareous tube attached to firm substrata. They tend to be a little too small for use in introductory invertebrate zoology laboratories some are tiny. Serpula vermicularis, a common shallow-water species along the entire west coast of North America, is large enough to be used in undergraduate teaching laboratories. This exercise is written for this species but will apply equally well to other serpulids. Hydroides dianthus, common on the east coast, is smaller. Because of the small size of serpulid worms that the exercise be confined to external anatomy.
Many features of serpulid and sabellid anatomy are similar and both are tubiculous, benthic, suspension feeding, sedentary worms. The two taxa are closely related and both belong to Sabellida. In both the prostomium and peristomium are fused to form a head which bears an elaborate branchial crown used for feeding and gas exchange. Both have a distinct thoracic region in which the positions of the two characteristic types of setae are opposite those of the abdomen. Both have a ventral abdominal fecal groove that runs across the right side to become a dorsal groove in the thoracic region. In both there is a single pair of metanephridia and they are anterior with an anterior nephridiopore. The hemal and nervous systems are similar.
Serpula inhabits a sinuous, white, calcareous tube attached firmly to the surface of hard substrata (Fig 13-51B). If you have a living worm still in its tube, take advantage of the opportunity to study it in this condition. Place it in a culture dish of seawater on the stage of your dissecting microscope and focus on the opening, or aperture, of the tube. Avoid bumping the desk or microscope and wait for the worm to emerge. The worm should eventually extend the branchial crown from the aperture of the tube.
The crown is composed of numerous slender featherlike radioles (also known as branchiae or gills), in two bundles, one right and one left (Fig 13-51B). They are attached to the anterior end of the head and are the feeding and respiratory organs. One of the radioles is modified to form an operculum that is used to seal the aperture of the tube when the worm is retracted. The operculum is a long stalk with a plug at the distal end. Sabellids, such as Eudistylia which is covered in another chapter, have radioles but no operculum.
>1a. Try adding a little carmine/seawater to the water in front of the branchial crown and behind it as well. Can you see evidence of transport of particles across the crown? In which direction? What do you suppose generates the current? What is the function of the current? <
>1b. See which environmental stimuli can cause retraction of the branchial crown. Does mechanical disturbance initiate retraction? Is retraction rapid? Do you see any evidence of eyes? Does the animal seem to respond to light or shadow presented independently of mechanical stimulation? <
1. If your specimen is still in its tube, use a screwdriver or table knife to break the tube away from the substratum without damaging the worm inside. The tube will break into fragments when you do this. Working under magnification and in fluid (magnesium chloride if alive, tapwater if preserved) carefully remove the worm from the broken tube, chipping away bits of tube as necessary to remove the worm. Notice that most of the tube has no floor and is an archway with the substratum which serves as the floor. Place the extracted worm in a small dissecting pan or culture dish of magnesium chloride (if living) or tapwater (if preserved) as appropriate and study it on the stage of the dissecting microscope.
When the worm is relaxed or nearly so, examine it with low power of the dissecting microscope. The body is a pretty combination of orange, salmon, red, and cream. It is divided into head, thorax, and abdomen. The head and branchial crown are anterior. The easiest way to distinguish dorsal from ventral is to find the dorsal operculum. The operculum is a long stalk with a plug at the distal end.
The head consists of the fused prostomium and peristomium and bears the branchial crown which resembles a feather duster. It is, of course, the anterior end of the body. The branchial crown, for which the animals are given the name fanworms, is composed of two clusters of featherlike radioles, one right and one left. Each cluster consists of a single row of radioles attached to a branchial stalk and curved in a semicircle. The two semicircles form the funnel-shaped branchial crown resembling a circular lophophore and functioning much like one. The mouth is at the apex of the funnel between the two branchial stalks. It lies between large upper (dorsal) and lower (ventral) lips. The lips are flat sheets of tissue extending transversely across the anterior end of the head from the base of one branchial stalk to the other. On each side a food groove runs from the bases of the radioles between the two lips to the mouth.
The radioles are like those of sabellids in being bipinnately branched (Fig 13-52). Each consists of a central axis from which arise short lateral branches, or pinnules, in a pinnate pattern. The central axis bears a median, ciliated longitudinal radiolar food groove and the medial surfaces of the pinnules are ciliated.
One of the radioles is highly modified to form the operculum which is used to plug the aperture of the tube when the animal withdraws. The operculum is dorsal. The operculum has a thick opercular stalk, and in this species, a soft hollow cone at its distal end. It takes other forms in other species. It is often a solid plug with chitinous spines distally, as in Hydroides . Sabellids, which resemble serpulids in many ways, have no operculum.
A single median nephridiopore is located dorsally on the head. It is very difficult to demonstrate but is located between the upper lip and a median dorsal papilla. The anterior end of the fecal groove passes over it and urine is released from it into the waste current. Serpulids, like sabellids, have only two nephridia. They empty via this nephridiopore.
The thoracic region of the body consists of seven chaetigers (= segments that bearing chaetae). The first is a collar segment and the head is attached to it. It bears an elaborate, delicate, membranous collar that overlaps the margins of the aperture of the tube and covers the opening of the tube when the head is extended. The collar extends from the ventral and lateral margins of the collar segment but is not present dorsally. Its gland cells secrete the calcareous tube.
On each side the collar turns and extends posteriorly to form a thoracic membrane running the length of the thorax. The membrane arises dorsally at the anterior end of the thorax and extends obliquely across the side of the thorax to end at the ventral midline. Sabellids, while having a well-developed collar, lack the thoracic membrane.
The thoracic membrane is dorsal to the parapodia. As in sabellids, the parapodia are reduced to low fleshy lobes but are well supplied with chitinous chaetae. The parapodia are divided into a dorsal branch, the notopodium and a ventral branch, the neuropodium. In the thorax each notopodium bears a bundle of long hairlike chaetae, called capillaries, whereas the neuropodium has a vertical row of short hooklike chaetae, called uncini. The collar segment has only the notopodium and notochaetae.
There is a median, longitudinal, ciliated, thoracic fecal groove on the dorsal midline of the thorax. It is a broad, shallow, relatively indistinct trough running the length of the thorax and ending at the head.
The remainder of the body is the abdomen, which is composed of numerous very short, wide segments. The terminal body region is the tiny pygidium. The anus is on the pygidium.
The abdominal parapodia are low and obscure but are long. Abdominal notopodia are long, vertical, fleshy ridges, each bearing a row of short, hooklike uncini which are easiest to see where light reflects off them. The notopodia extend from the middle of the side of their segment to the dorsal midline. The neuropodia are much smaller and are located at the ventral (lateral) end of the notopodium. Each bears a small tuft of short, inconspicuous, bristelike capillary chaetae. If the chaetae are retracted, they will be difficult or impossible to see.
Note that, as in sabellids, the positions of the uncini and capillaries are reversed in thorax and abdomen.
The midventral abdominal fecal groove extends the length of the ventral midline of the abdomen. It connects by an oblique lateral fecal groove with the dorsal thoracic fecal groove which you saw earlier. The lateral groove is on the right side of the body at the junction between thorax and abdomen.
Vertical ciliary tracts in the grooves between adjacent abdominal segments move particles dorsally in the dorsal half of the body and ventrally, to the abdominal fecal groove, in the ventral half of the body. There is a weak anteriorly directed current along the dorsal abdominal midline. Together these several ciliary currents transport particulate matter (feces, gametes, detritus) out of the depths of the tube to the aperture where it can be released to the sea. The only major region of the body without these housekeeping currents is the ventral thorax.
>1c. Apply some carmine/seawater suspension to various parts of the body surface of your relaxed specimen and demonstrate to yourself the existence, direction, and efficacy of the housekeeping currents. <
Each abdominal segment has a pair of tiny gonopores. These are situated on either side of the ventral midline but you probably will not see them unless they release gametes while you watch. Recently captured specimens often shed gametes so this is a possibility.
>1d. If specimens in the laboratory are shedding gametes, mix eggs and spermatozoa in a dish of clean seawater and follow the early stages of spiral cleavage. <
Ruppert EE, Fox RS, Barnes RB. 2004. Invertebrate Zoology, A functional evolutionary approach, 7 th ed. Brooks Cole Thomson, Belmont CA. 963 pp.
Thomas JG. 1940. Pomatoceros, Sabella, and Amphitrite. Liverpool Mar. Biol. Comm. Mem. 33:1-88, pls 1-11.
Slides and coverslips
Glycera, about 10-12 cm
Dissecting pan. Sardine, anchovy fillets, or smoked oyster tins poured with wax are a good size.
# 1 stainless steel insect pins
Isotonic magnesium chloride if using living Glycera
Dissecting set with microdissecting tools