43^ GEPHYREA 



CHAP. 



Gephyrea Armata to the above -mentioned genera. In addition 

 to these, Echiurus has a row of chitinous bristles surrounding 

 the posterior end of the body ; the row is single in E. unicinctus, 

 double in E. pcdlasii. These bristles are formed, like the hooks 

 on the introvert of the Sipunculoidea, by epidermal cells ; those 

 of B. minor and of the posterior rings in Echiurus are said to 

 arise each from a single cell, just as the bristles do in Chaetopods. 



The skin consists of very much the same layers as does that 

 of SipuTiculus ; the cuticle is thin, the epidermis is modified into 

 numerous glandular cells, papillae, and pits, from which the 

 bristles arise. A third layer of oblique or circular fibres is 

 usually found inside the longitudinal muscle-layer. The proboscis 

 is solid, and contains much connective-tissue and numerous muscle - 

 fibres running in all directions ; the ventral groove is ciliated. 



The alimentary canal in the Echiuroidea consists of a long 

 thin-walled tube with numerous convolutions ; it is not coiled as 

 in Sipunculids, but the loops are irregularly arranged, and are 

 supported by numerous fine muscular strands which run from the 

 skin. There is a ciliated groove running along one side of the 

 intestine, as in the Sipunculids. The anus is terminal. The 

 most striking peculiarity of the alimentary canal of the Echiurids 

 is the existence of a collateral intestine or " siphon." This is a 

 narrow tube which arises from the main canal not very far 

 from the mouth, and re-enters it again lower down. A similar 

 structure occurs in some Echinids, and in the Capitelliformia 

 (pp. 272, 305). Its function is not certainly known. 



Another characteristic feature of the Echiurids is the presence 

 of " anal vesicles," branching structures which unite into a common 

 stem opening into the intestine close to the anus. The free end 

 of each of the branches terminates in a ciliated funnel-shaped 

 opening. The function of these structures may be excretory, or 

 they may control the amount of fluid in the body-cavity. 



A closed vascular system exists in Echiurids, consisting of a 

 contractile dorsal vessel running along the dorsal surface of the 

 anterior end of the alimentary canal, and continued along the axis 

 of the proboscis. At the tip of the proboscis it bifurcates, and 

 each branch descends along the edge until it reaches the base where, 

 having encircled the oesophagus, the two unite, and are continued 

 as the ventral vessel which runs along the dorsal surface of the nerve- 

 cord, and eventually ends blindly. There is also a vessel which 



