SCAPHOPODA. 



351 



Sense organs. Eyes are absent. A pair of otocysts are placed on 

 the pedal ganglia. The tentacles serve as tactile organs. 



The alimentary canal is divided into a buccal cavity, oesophagus 

 with two lateral pockets, stomach with large liver, and an intestine, 

 which after several coils closely pressed together, opens behind the 

 foot and the visceral commissure into the 

 middle of the mantle cavity. The bnccal 

 cavity is placed in the body at the base 

 of the cephalic projection, and contains a 

 dorsal jaw and a ventral radula, which 

 has a short sac and powerful muscles. 



The vascular system is reduced to two 

 mantle vessels and a complicated system 

 of wall-less spaces throughout the body. 

 There are no specialized respiratory 

 organs. 



The excretory organs are paired and 

 lie in the middle region of the body. 

 They open on either side of the anus. 



The Scaphppoda are dioecious. The 

 ovaries and testes are unpaired organs 

 occupying the posterior part of the body 

 behind the liver and intestine. They 

 open into the right kidney. 



The animals live buried in mud, and 

 creep about slowly by means of their 

 foot. There are about 100 species scat- 

 tered in all seas, from the littoral to a 

 depth of about 2000 fathoms. They are 

 known since the Devonian. 



6 



Development. The eggs are laid singly. There 

 is an invagiuate gastrula with a large blastopore, 

 which is at first at the hind end. The embryo 

 elongates and the hinder part of the body grows 

 out behind the blastopore, which becomes the 

 month. The free-swimming larva has a preoral 

 region with a ciliated tuft and several circles 

 of cilia, which eventually consolidate into the 

 velum. The mantle arises as two dorso -lateral folds which eventually coalesce 

 ventrally. The shell also is at first bivalve, but subsequently becomes tubular. 



The Scapliopoda are allied to the Lamellibranchs by their mantle 

 and nervous system, but they possess an odontophore which approxi- 



a 



FIG. 274. Larva of Dcntalium 

 (after Lacaze Duthiers). a, 

 young larva with first rudi- 

 ment of shell. 1>. older larva 

 seen from the dorsal surface. 

 T tentacle-collar ; MT circular 

 muscle of mantle ; P foot ; Gg 

 cerebral ganglion ; Oes oeso- 

 phagus ; L liver ; DM buccal 

 mass ; S rudiment of shell. 



