Invertebrate Anatomy OnLine

Ligia exotica ©

Sea Slater


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 on 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, Eumalacostraca, Caridoida, Xenommacarida, Nomen nominandum, Neocarida, Peracarida SO, Isopoda O, Onsicoidea sO, Ligiidae 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.

Peracarida SO

            Peracarida is a large and ecologically important malacostracan taxon.   It includes the amphipods and isopods, each with well over 5000 species, as well as several smaller taxa such as mysids, cumaceans, and tanaidaceans.   Peracarids have the characteristics of Malacostraca and are further defined by possession of a ventral, thoracic marsupium, or brood pouch, in which the eggs are brooded.   Development is direct and there is no larva.   The mandible bears a movable tooth, the lacinia mobilis, between the molar and incisor. Most peracarids are small, less than 2 cm and primarily inhabit marine and freshwater habitats, although some are terrestrial.  

Isopoda O

            Isopoda is a large and diverse taxon with nine high level subtaxa.   Isopods are common in marine and freshwater habitats and there are important terrestrial and parasitic suborders.   It includes the familiar terrestrial pill bugs, wood lice, and sea slaters, and about 4000 less familiar marine species.   Isopods are usually dorsoventrally flattened and the gills and heart are abdominal.   One thoracic segment is fused with the head and there is, correspondingly, one pair of maxillipeds and seven pairs of thoracic walking legs.   The gills and heart are abdominal.

Oniscoidea sO

             Oniscoideans are the terrestrial and semiterrestrial isopods and the most successful terrestrial crustaceans. These are the woodlice, sowbugs, pillbugs, and sea slaters. The first antenna is tiny and the mandible has no palp. The pleon consists of five free pleomeres plus a pleotelson consisting of the sixth pleomere fused with the telson. The exopods of some pleopods have pseudotracheae and are modified for respiration in air.  

Laboratory Specimens

            In coastal locations the sea slater, Ligia, is a good choice for the study of peracaridan anatomy because it is usually the largest readily available peracaridan. Ligia species are also known as wharf roaches or boat roaches because they resemble those insects in size, shape, and agility. Ligia are strictly a coastal and are not found inland.  Although coastal, it is not a marine animal.   It lives on the edge of the sea in the supratidal zone and ventures into the intertidal only when the tide is out.   It belongs to the terrestrial suborder Oniscoidea which also includes the woodlice and pill bugs, common in gardens and yards under stones or wood.  

            At least one species of Ligia is present on most coasts. The several species of Ligia differ little from each other and this exercise can be used with any of them.   Ligia exotica is a widespread European species thought to have been transported by wooden sailing ships to both coasts of North America. It occurs on the eastern North American coast from Chesapeake Bay south to the Caribbean and on the west coast from California to Chile.   Ligia oceanica, also a European species, occurs from Cape Cod north to Maine.   Ligia baudiniana is reported from South Florida to Brazil, Bermuda, and the Caribbean. On the west coast Ligia occidentalis is found from Central California south to Central America.

            The study can be conducted using living (relaxed) or preserved specimens. If you are using a living specimen, relax it by placing it in isotonic magnesium chloride.   These are very active animals and it is necessary that they be relaxed.   The specimen should be in a small dissecting pan immersed in the appropriate fluid (magnesium chloride if living, tapwater if preserved).                   

External Anatomy


            Examine the dorsum first. The tagmata are indistinct but cephalothorax, thorax, and abdomen are present.  

            The cephalothorax consists of five head segments plus one thoracic segment but there is no external indication of this segmentation (Fig 1, 19-61).   Dorsally and laterally it bears a pair of large compound eyes.   The eyes of isopod and amphipods are not stalked, and are said to be sessile.  

            The malacostracan thorax includes eight segments, or thoracomeres, but in isopods the first one is fused with the head to make the small cephalothorax.   The remaining seven thoracomeres are the pereon and each is a pereomere.   They are independent of each other and of the head.   There is no carapace.

            The pereomeres are good examples of typical arthropod segments. Each is covered by an exoskeletal ring composed of two sclerites connected laterally by the flexible articular membranes of the pleurites (Fig 16-1B).   The dorsal sclerite is the heavily sclerotized, arched tergite (Fig 1).   The seven tergites of the pereon are easy to see and count. The ventral portion of each segmental exoskeletal ring is the sternite.   It is also sclerotized but less so than the tergite.   Laterally each tergite is produced into an epimeron, or side plate, that enhances the overhand of the tergite and partially encloses the ventral space under the pereon (Fig 1).  

            The abdomen, also known as the pleon, has six segments, or pleomeres (Fig 1).   The posterior telson is fused with the sixth pleomere to form the pleotelson.   Like most isopods,Ligia is dorso-ventrally flattened (depressed).


            Begin your study of the appendages with the cephalothorax.   The characteristic five pairs of head appendages and one thoracopod are present.   The first antennae are vestigial in Oniscoidea and are barely visible at the medial bases of the much larger second antennae (Fig 1).   Like insects, terrestrial isopods have only one functional pair of antennae.  

            The second antennae are large and conspicuous.   They are attached to the anterior surface of the head and are uniramous.   Each consists of a proximal peduncle of five articles and a long distal flagellum of many small articles (Fig 1).  

            The remaining head appendages are three pairs of mouthparts.   They, along with the appendage of thoracomere 1, are located in a conical protuberance on the median ventral surface of the head called the oral cone.   The first thoracomere, alone among thoracomeres, is fused with the head and its appendages are the maxillipeds.   The maxillipeds form the posterior wall of the oral cone and cover the mouthparts.  

            The mouthparts may be studied in place or you may remove them to make a wetmount for examination with the compound microscope.

Figure 1 Dorsal view of the sea slater, Ligia exotica. Segments of the antenna 2 peduncle, the pereon, and the pleon are numbered. Redrawn from van Name (1936). Isopod74L.gif

Figure 1

"     To remove the mouthparts, begin posteriorly and remove the maxillipeds first and then proceed anteriorly.   This is easy to do if you hold the animal on its back with a minuten nadel in one hand.   Pierce the anterior end of the ventral surface with the nadel and pin it down against the dish.  With your finest forceps in the other hand, grasp each appendage in turn and twist it off the oral cone and place it in a drop of water on a slide.

            Being thoracic limbs the maxillipeds have the same basic structure as a typical malacostracan thoracopod (Fig 2).   Accordingly, the maxilliped is a modified endopod, with the usual seven articles.   The basal article is the coxa, with which articulates a large basis, followed by a five-segmented palp consisting, in order, of the ischium, merus, carpus, propodus, and dactyl. A large medial endite extends distally from the basis and an epipodite is on the lateral border of the coxa. The straight medial borders of each maxilliped bear short coupling hooks used to link it to the other maxilliped so they function as a unit.  

Figure 2. The maxilliped of Ligia.   Redrawn from van Name (1936). Isopod71L.gif

Figure 2

            The small biramous second maxilla is anterior to the maxilliped (Fig 3).  Its endopod is inconspicuous and the exopod a little larger. The first maxilla, also biramous, is larger than the second maxilla and consists of an exopod and endopod (Fig 4).   The soft, fleshy, lobed paragnath, or lower lip, lies just anterior to the first maxilla.   It is a fold of the body wall and is not a segmental appendage.  

            Anterior to the paragnath are the mandibles (Fig 5).   Each is sclerotized and brittle and thus easily crushed but should be removed intact if possible. The two mandibles lie on the right and left sides of the mouth and the other mouthparts are posterior to the mouth. It has apical dark brown incisor for cutting and a basal molar for grinding.   A movable toothed process, thelacinia mobilis, is present on the mandible between the incisor and molar. The mandible of isopods and amphipods usually has a 3-segmented palp, but Ligia, being an oniscoidean, has no mandibular palp.

            Look at the appendages of the pereon.   There are seven pereomeres and seven pairs of pereopods, or walking legs.   The name "Isopoda" refers to the fact that in most isopods, including Ligia, the seven pereopods are similar to each other.  

Figure 3. The second maxilla of Ligia. Redrawn from van Name (1936). Isopod69L.gif

Figure 3

            Choose one of the pereopods for further study.   The pereopods have the basic seven articles of the malacostracan thoracic appendage.   The proximal coxa, however, is fused with the epimeron, extending the overhang of the tergite even farther from the body.  

Figure 4. The first maxilla of Ligia. Redrawn from van Name (1936). Isopod73L.gif

Figure 4

            Articulated to the coxa is a long basis, followed by a shorter ischium (Fig 6).   Next are the merus, carpus, propodus, and dactyl in that order.   The short orange dactyl of this species is bifid, that is, it ends with two small teeth.

            Female specimens may have medial oostegites, or brood plates, on the coxae of the first five pairs of pereopods (Fig 19-52, 19-62B).   The ten oostegites form a marsupium, or brood pouch, that receives and broods the eggs after they are released and fertilized.   The marsupium is located under the pereon.

Figure 5 The mandible of Ligia. Redrawn from van Name (1936). Isopod70L.gif

Figure 5

            The male reproductive tract opens, as in all Malacostraca, on the ventral surface of the eighth thoracomere (7 th pereomere).   They open at the end of a pair of small soft papillae, thepenes, medial to the coxae of the 7 th pereopod.   The penes are not intromittent organs.   The female gonopores are on the 6 th thoracomere (5 th pereomere).


Fig 6 Pereopod 1 of a male Ligia. Redrawn from van Name (1936). Redrawn from van Name (1936). Isopod72L.gif

Figure 6

            The pleon, or abdomen, is six segmented.   The anterior five pleomeres bear biramous appendages.  Their endopods and exopods are broad, flat, and leaflike.   The pleopods are the gills and their rami are the slater's respiratory surface.   They are soft and pliable with a thin permeable cuticle.  

            In some isopods the first pair of pleopods forms a heavily cuticularized operculum to cover and protect the delicate gills but that is not the case in Ligia.   There is no protective operculum.

            In males the medial rami (endopods) of the first and second pleopods are modified to function in sperm transfer.                                                              

            The telson is fused with the sixth pleomere to form a pleotelson (Fig 1).   The sixth pair of abdominal appendages is the uropods attached posteriorly to the pleotelson.   They are biramous with a narrow, basal protopod and two long pointed rami.   The median endopod is a little longer than the lateral exopod.  

            The anus is a ventral vertical slit on the pleotelson under the overhang of the telson.


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

            Snodgrass, R. E .   1952.   A Textbook of Arthropod Anatomy.   Cornell, Ithaca.   363p.  

            Sutton SL.   1972.   Woodlice.   Pergamon, Oxford.   144p.

            Van Name, W. G.   1936.   The American land and fresh-water isopod Crustacea.   Bull. American Mus. Nat. Hist.   71:1-535.

            Wägele J-W.   1992.   Isopoda.   in Harrison, F. W. &   A. G. Humes (eds.).   Microscopic Anatomy of Invertebrates vol. 9 Crustacea.   Wiley-Liss, New York.


Dissecting microscope

Small wax-bottom dissecting pan

Living or preserved Ligia

Isotonic magnesium chloride for living specimens

# 1 stainless steel insect pins