Invertebrate Anatomy OnLine

Nebalia pugettensis ©


Copyright 2001 by

Richard Fox

Lander University


            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, Thoracopoda, Phyllopodomorpha, Ostraca, Malacostraca C, Leptostraca O, Nebaliacea sO, Nebalidae F , (Fig 16-15, 19-67, 19-90)

Arthropoda P

            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 sP

            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.


            In the ancestral thoracopod the thoracic appendages were turgor appendages used for suspension feeding in conjunction with a ventral food groove. Such appendages and feeding persist in several Recent taxa but have been modified in many others.


            The compound eyes are stalked primitively although derived sessile eyes occur in many taxa.

Malacostraca C

            Malacostraca includes most of the large and familiar crustaceans such as crabs, shrimps, lobsters, crayfish, isopods, and amphipods.   Primitively the trunk consists of 15 segments, eight in the thorax and seven in the abdomen but in most Recent species the abdomen has only six segments. The female gonopore is on the eighth thoracic segment and the male on the sixth.

Leptostraca O

            The 20 odd known leptostracans, or phyllocaridans, are small benthic marine malacostracans. Recent leptostracans are considered to be relics of a larger Cambrian taxon.    The enormous bivalve carapace encloses the thorax and most of the abdomen.   The compound eyes are stalked. No thoracic segments are fused with the head and there is thus no cephalothorax and no maxillipeds. The eight thoracopods are biramous phyllopods.   The rostrum has a small anterior region, the rostrum. The abdomen is seven, not six, segmented.   The anterior abdominal appendages are biramous and well developed natatory legs.   The posterior abdominal appendages are vestigial or absent. The large telson bears the anus and caudal furca. The heart is a long multiostiate tube extending most of the length of the animal.   The animals are gonochoric and the gonads are long tubes.   The long nerve cord bears 17 ganglia.

Laboratory Specimens

            Nebalia bipes occurs in vegetated areas of the northeastern coast and in Europe.   Nebalia pugettensis is common on the west coast of North America in algae and mud near the low tide line.   Nebalia is also found in the Florida Keys.   The differences between the species are minor and any can be used for this study.   Living or preserved specimens can be used.  

            Conduct the study in liquid in a small glass culture dish.   Living specimens should be in isotonic magnesium chloride.   Preserved material should be in tapwater.  

            Use the dissecting microscope with transmitted light.   The exercise can be accomplished without dissection of the appendages if desired.   For a careful study of appendage morphology, however, removal is necessary.   Because of the small size of the animals the study is limited to external anatomy.

External Anatomy

             The body consists of the usual crustacean tagmata.   There is an anterior head, middle thorax, and posterior abdomen.   No thoracic segments are fused with the head and there is no cephalothorax.


            The head consists of five fused segments bearing the five pairs of head appendages characteristic of crustaceans. The dorsal body wall of the posterior head is extended posteriorly to form a very large two-part carapace (Fig 1, 19-21).   The small anterior part is hinged to the larger posterior part and forms a movable visor over the anterior head and eyestalks.   It is used to control water currents.   It is usually referred to as a " rostrum" but does not resemble the rostrum of other crustaceans which is an unhinged anterior projection of the head.

            The posterior part of the carapace is very large (Fig 1, 19-21).   It is bivalved, being composed of large right and left valves which enclose most of the animal between them.   The valves are joined dorsally by a hinge.   The valves are moved toward each other by a transverse adductor muscle (Fig 1) running from one valve to the other.   Elastic recoil of the hinge restores the valves to their open position.

            Anteriorly the head bears a pair of large, stalked compound eyes.  Posterior to the eyes are the large first antennae (Fig 1, 2).   Each consists of a large peduncle bearing two rami.  The rami arise from the fourth article of the peduncle.   One ramus very short and consists of a single article whereas the other is a multiarticulate whiplike flagellum.  

            The remaining head appendages are the three pairs of mouthparts.   These are difficult to discern without removing one valve of the carapace.   Dissection of the mouthparts is optional.

Figure 1.   Side view of a female Nebalia pugettensis from Fossil Point, Coos Bay, Oregon.   Nebalia1L.gif

Figure 1

            The second antennae are larger than the first and bear a single whiplike flagellum.   The second antenna of males is much longer than that of females and is about the length of the body whereas that of females is much shorter than the body.  

Figure 2.   First antenna of Nebalia pugettensis. Nebalia2L.gif

Figure 2

            Figure 3.   Second antenna of Nebalia pugettensis.   nebalia3L.gif nebalia3L.gif

Figure 3

            The paired uniramous mandibles lie on either side of the mouth (Figs 1, 4).   Each mandible consists of a basal portion which bears a protuberant, ridged molar.   The molar faces the molar of the other mandible.   A 3-articulate mandibular palp extends from the basal portion.   The third article of the palp bears a comblike fringe of setae.   You may be able to see this terminal article extending beyond the ventral edge of the carapace (Fig 1).

Figure 4.   The mandible of Nebalia pugettensis.   Nebalia4L.gif

  Figure 4

            The uniramous first maxillae are posterior to the mandibles.   Each consists of a basal portion from whose lateral surface extends a very long, slender maxillary palp (Figs 1, 5).   The palp extends posteriorly at an upward sloping angle beside the body between the thorax and the carapace (Fig 1).   Its function is to keep the interior of the carapace clean.  

Figure 5.   Maxilla 1 of Nebalia pugettensis. Nebalia5L.gif

Figure 5

            The biramous second maxillae are the last of the mouthparts (Fig 6).   Each has a large basal portion bearing three setose endites and two distal rami, one of which is the exopod, the other the endopod.

Figure 6.   Maxilla 2 of Nebalia pugettensis. Nebalia6L.gif

Figure 6


            The thorax consists of eight segments as it does in all malacostracans.   Each thoracic segment bears a pair of biramous appendages. The male gonopores are on thoracic segment 8, the female on 6, as expected in malacostracans.   The thoracic segments (or thoracomeres) are very short but the grooves separating them are visible through the transparent carapace, especially with transmitted light (Fig 1).   (The carapace tends to become translucent or opaque after death).   The adductor muscle may be visible just anterior to the groove separating the first thoracic segment and the head.   It extends transversely between the two valves.

            There is little variation in the eight pairs of thoracic appendages (thoracopods).   All the thoracomeres are independent of each other and of the head and none of their appendages is modified to form a maxilliped.

            Each thoracopod consists of a basal protopod attached proximally to the ventral surface of the thorax (Fig 7).   From it arise an attenuate, medial, setose endopod, a large, flat, proximal epipod, and a smaller, flat, distal exopod.  The epipod does not bear setae.


            The anterior six abdominal segments each bear a pair of biramous appendages but the seventh has no appendages.   The telson is located on the seventh segment and itself bears acaudal furca with long rami, unusual in malacostracans but common in other crustacean groups.   The anus is also on the telson.

            The abdomen consists of seven cylindrical segments, or pleomeres, and a telson. This is one more pleomere than the typical malacostracan condition.   The first four or five segments are covered by the carapace.  

            Segments 1-6 each bear a pair of pleopods whereas segment 7 has no appendages.   Pleopods 1-4 are large and biramous (Fig 8).   These consist of a long, robust basal article from which arise two slender rami.   The endopod is spinous and the exopod is setose.   All four pairs are similar.  

Figure 7.   Thoracopod 1 of Nebalia pugettensis.   Nebalia7La.gif

Figure 7

            The pleopods of abdominal segments 5 and 6 are small and uniramous (Fig 1).

The telson extends posteriorly from segment 7 and bears a caudal furca with two rami.   Each consists of a long spiny article from which arise two very long setae (Fig 9).  

            The anus is located at the distal end of the telson between the two furcal rami (Fig 9).   With careful focusing you may see the short rectum (hindgut) in the telson.  

Figure 8.   Pleopod 1 of Nebalia pugettensis. Nebalia8L.gif

Figure 8

Figure 9.   Dorsal view of the telson and caudal furca of Nebalia pugettensis.   Nebalia9L.gif

  Figure 9


            Abbott DP .   1987.   Observing Marine Invertebrates.   Stanford Univ. Press, Stanford.   380p.  

            Bullough. WS .   1958.   Practical Invertebrate Anatomy (2 nd   ed).   MacMillan, London.   483p.  

            Ho J-S .   1978.   Laboratory Manual for Invertebrate Zoology Emphasizing Marine Forms .   Hwong, Los Alamitos, Calif.   152p.

Ruppert EE, Fox RS, Barnes RB.   2004. Invertebrate Zoology, A functional evolutionary approach, 7 th ed. Brooks Cole Thomson, Belmont CA. 963 pp.  


Dissecting microscope

8-cm culture dish

Living or preserved Nebalia

Isotonic magnesium chloride for living specimens