VERMES : ENTEROPNEUSTA. 589 



hairs are restricted to the dorsum. The locomotor rods are very slende"r and 

 hooked, situated so far anteriorly that the vagina opens among them. 



Chaetosoma and Rhabdogaster, Metschnikoff, Z. W. Z. xvii. 1867. Tristico- 

 chaeta, Panceri, op. cit. supra, p. 7. 



CLASS ENTEROPNEUSTA. 



Marine Vermes with a ciliated epidermis, and a body divided into three 

 regions, viz. a prae-oral contractile proboscis, a ' collar* surrounding the narrow 

 base of the proboscis, and a long worm-like trunk. The first portion of this trunk 

 is pierced by a double series of dor sally placed respiratory pores : the middle 

 portion bears dorsal lobes formed either by prominent sexual glands, or sexiial 

 glands mingled with liver caeca ; and the hind portion is distinctly ringed. 

 The moiith is ventral at the base of the proboscis in front of the collar ; the 

 anus terminal and widely open, being devoid of a sphincter. There is but one 

 genus, Balanoglossus, with several species from different parts of the world. 

 The animals exhale peculiar and characteristic odours ; are brightly coloured, 

 the males differing in tint from the females ; and live immersed in mud and 

 sand, which they saturate with mucus. This mucus sometimes sets firmly, as 

 in B. Robinii. Some species are phosphorescent. 



The epidermis consists of a single (?) layer of cells, some with a 

 delicate cuticle and cilia, others unicellular mucous glands J . The body- 

 cavity is divided into five sections, an anterior within the proboscis, two 

 paired (right and left) in the collar, and two similarly paired in the trunk, 

 derived respectively from as many outgrowths of the archenteron. The 

 outer cellular walls of these outgrowths form the body-walls, composed of 

 connective and non-striated muscular tissues. The latter is arranged as 

 a well-developed longitudinal coat. In B. minutus there is also a delicate 

 external circular coat, wanting in other species. The longitudinal coat 

 consists of several concentric layers in the proboscis. The original cavity 

 of the proboscis is filled by a network of stellate cells in the adult ; their 

 interspaces communicate with the exterior by a ciliated (epiblastic) canal 

 opening at the base of the proboscis in the middle dorsal line, or on the 

 left side (B. Kowalewskii), or by -two such canals (B. Kupfferi). The 

 inner walls of the archenteric outgrowths in the collar and trunk form the 

 dorsal and ventral mesenteries, and the lateral transverse muscles of the 

 digestive canal which are interrupted in the line of the mesenteries. The 

 dorsal mesentery is in some species absorbed from place to place in the adult. 

 The cavity or cavities (right and left) in the collar may be much oblite- 

 rated by growths of connective tissue, but communicate with the exterior 

 by a right and left ciliated (epiblastic) canal opening into the first gill-slits, 



1 Bateson thinks that the second layer of epidermic cells, often supposed to exist, is due to the 

 breaking off in teased preparations of the lower ends of the long cells, Q. J. M. xxvi. p. 513. 



