Invertebrate Anatomy OnLine

Anoplodactylus lentus ©

Sea spider


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, Chelicerata sP, Pycnogonida C, Phoxichilidiidae F (Fig 16-15, 18-47)

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.

Chelicerata sP

            Chelicerata is a large taxon that includes spiders, scorpions, pseudoscorpions, ticks, mites, horseshoe crabs, sea spiders, and many others.   The group originated in marine habitats but almost all modern chelicerates are terrestrial.  

            The body is divided into an anterior cephalothorax with six pairs of appendages and a posterior abdomen which, in most groups, does not bear appendages or has highly modified appendages.   The first appendages of the cephalothorax are the chelicerae.   Antennae are not present and the brain has two regions rather than the three found in mandibulates.   Appendages are primitively biramous but are uniramous in almost all Recent taxa.

Pycnogonida C

            Pycnogonids (= Pantopoda), or sea spiders, are benthic, marine arthropods, with a superficial resemblance to true spiders, but to which they are probably only distantly related.  Pycnogonids are carnivores and use a muscular pharynx to suck soft food into the gut.   The cephalothorax is segmented and lacks a carapace and the abdomen is vestigial. Four median eyes are located on a prominent tubercle.   The body is tubular.

            The mouth is borne at the end of a large proboscis, an unusual feature for an arthropod.   Digestion is intracellular and most feed on sponges, cnidarians, or bryozoans from which they suck fluids.   Saccate nephridia have recently been found in one species. No respiratory organs or coelom are present.  

            The nervous system is typically arthropodan with dorsal brain, circumesophageal connectives, and a double, ventral, longitudinal nerve cord with segmental ganglia.   Like that of other chelicerates, the brain is two-part and lacks the deutocerebrum.   The first pair of appendages is a pair of pincer-like chelicerae and the second pair is the sensory pedipalps.   A dorsal tubular heart pumps blood to the hemocoel.  

            Sea spiders are gonochoric, fertilization is external, and there is a slight tendency to sexual dimorphism.   The larva is a protonymphon with three pairs of appendages and a blind gut.

Laboratory Specimens

            This exercise is written for the common, relatively large, east coast pycnogonid, Anoplodactylus lentus , but notes are included for other genera. Pycnogonida is sufficiently homogeneous that the exercise can be used with any species.   Anoplodactylus lentus lives on the hydroid, Eudendrium carneum, which is its prey.   Because of the small size, dissection and study of the internal anatomy is impractical.

External Anatomy


            Place your specimen in a small finger bowl of seawater if living, or tapwater, if preserved.   Study the external anatomy with the dissecting microscope.

            Pycnogonid tagmosis differs somewhat from that of other chelicerates and three tagmata, cephalon, trunk, and abdomen, can be identified (Fig 1, 18-45) rather than the cephalothorax and abdomen of other chelicerates.   The typical chelicerate cephalothorax consists of a two-segmented head and four or five segmented thorax. In contrast, the pycnogonid cephalon consists of the two segmented chelicerate head fused with two anterior thoracic segments leaving the trunk with three (or more) thoracic segments.

Figure 1.   An oblique view of the left side of a male Anoplodactylus lentus from Murrell's Inlet, South Carolina.   The left legs have been removed to reveal the left oviger. Pycnog20L.gif

Figure 1


            The cephalon is the anterior end of the body.   It is superficially unsegmented but probably is formed of four fused segments for it bears four pairs of appendages (chelicerae, pedipalps, ovigers, and a pair of walking legs).   The cephalon is probably homologous to the head and first two thoracic segments of an ancestral chelicerate.  

            Anteriorly the cephalon terminates in an unpaired median proboscis with the mouth at its tip (Fig 2, 18-46).   In Anoplodactylus the proboscis is a long cylinder attached to the ventral side of the head by a flexible articulating membrane that allows it to be moved independently of the body.   The mouth is at the anterior tip of the proboscis.   The proboscis is used to penetrate the body wall or plasma membranes of the prey's body or cells.   The mouth opens into the muscular, pumping pharynx inside the proboscis (Fig 18-46A).   The size, shape, and mobility of the proboscis correlate with the type of food utilized by the species.

Figure 2. View of the left side of a male Anoplodactylus lentus. Pycnog21L.gif

Figure 2

            The cephalon of Anoplodactylus bears a pair of long, narrow chelicerae, or chelifores (Fig 2, 18-46A,B) which are the first pair of segmental appendages.   The chelicerae have a pincer, or chela, composed of two distal articles.   In Anoplodactylus the chelicerae are a little longer than the proboscis and are used to tear food and transfer it to the mouth.   (Tanystylum has vestigial chelicerae, those of Nymphon are well developed [Fig 18-45], and Pycnogonum has none.)   (The terms chelicerae and pedipalps are avoided by pycnogonid specialists to avoid emphasizing an as yet unconfirmed homology with the similar structures of other chelicerates. Instead, the terms “chelifore” and “palp” are used.)  

            The head of many pycnogonids also bears a pair of slender, many-jointed pedipalps, or palps (Fig 18-45), but these are absent in Phoxichilidiidae to which Anoplodactylus belongs.  (Nymphon and Tanystylum have pedipalps.   Pycnogonum has neither palps nor chelicerae.)   When present, the palps are attached laterally, at the base of the proboscis and have sensory and feeding roles.            

            The prominent, median ocular tubercle on the dorsal surface of the head (Fig 2, 18-46A) bears four simple eyes spaced evenly around its circumference so the animal can detect light for a full 360 ° .

            The cephalon has a pair of lateral projections with which a pair of long walking legs articulates.   In males, the posterior cephalon also bears a pair of ovigers , or ovigerous legs,which are used to carry the egg mass (Figs 1, 2, 18-45, 18-46B).   The walking legs and ovigers are probably homologous to thoracic appendages. Female Anoplodactylus, Pycnogonum, andTanystylum do not have ovigers but females of many pycnogonids do.   The ovigers are also used for grooming the other legs.   The ovigers arise at the base of the first pair of walking legs and are held below the body.   They are smaller than the walking legs.


            The trunk of Anoplodactylus consists of three free segments which together make up an exceedingly slender, sticklike body (Fig 1).   (Some pycnogonids, such as Tanystylum, have wide bodies.)  

            Each trunk segment (as well as the cephalon) bears a pair of lateral projections with which a pair of long walking legs articulates.   In addition to walking, the legs of Anoplodactylusand of many other pycnogonids can be used for swimming.

            Each walking leg consists of a series of nine articles and terminates in a prehensile pincer formed of the last two articles (Fig 1, 18-46B).   The pincer is used to grasp the substratum, which is frequently the narrow stem of a hydroid (or bryozoan).   The articles are, in order from proximal to distal: coxa 1, coxa 2, coxa 3, femur, tibia 1, tibia 2, tarsus, propodus, and claw (Fig 1). 

            Because of the lack of space in the tiny body, parts of some organ systems, such as the digestive and reproductive systems, are housed in the legs.   Diverticula (digestive ceca) from the gut extend far into legs and the female legs are expanded to provide room for the developing ova (Fig 18-46B).  

            The gonopores are on the legs but they are small and inconspicuous.   Pycnogonids differ from all other arthropods in having multiple, segmental gonopores. The hatching stage is a protonymphon larva with three pairs of appendages (Fig 18-46C).   Its appendages (chelicerae, pedipalps, and first walking legs) are those of the adult cephalon.


            The vestigial, unsegmented abdomen is a small dorsal process at the posterior end of the last trunk segment (Fig 1, 18-45).   The anus is at its tip.


            Arnaud F, Bamber RN .   1987.   The biology of Pycnogonida.   Advances Mar. Biol.   24:1-96.

            Hedgpeth JW.   1948.   The Pycnogonida of the western North Atlantic and the Caribbean.   U. S. Nat. Mus. Proc. 97:157-342.

            King PE.   1973.   Pycnogonids.   St. Martin's Press, New York.   144p.

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

Living or preserved sea spider

8-cm finger bowl

Isotonic magnesium chloride or sea water