XIII 



PHYLUM CHORDATA 



11 



be' 



have a feature in common with such a colony that they multiply by 

 the formation of buds ; but these become detached before they 

 are -mature. With the collar-region are connected a series of usually 

 eight to sixteen arms, each beset, except in the case of the male of 

 one species, with numerous very fine pinnately-arranged tentacles, 

 and containing a prolongation of the collar-cavity. The proboscis 

 (Fig. 722, ps.) is a shield- shaped lobe overhanging the mouth ; its 

 cavity communicates with the exterior by two proboscis-pores 

 (p. p.). The cavity of the collar communicates with the exterior by 

 a pair of ciliated passages opening by the collar-pores. Behind the 

 collar-region on each side is a small area in which the body-wall 

 and that of the pha- 

 rynx are coalescent ; 

 this area is perforated 

 by an opening - - the 

 gill-slit. Cilia occur 

 only on the arms, pro- 

 boscis, and lateral lips. 

 A nerve-strand, dorsal 

 ganglion, or collar-cord , 

 containing nerve-fibres 

 and ganglion-cells, is 

 situated on the dorsal 

 side of the collar below 

 the epidermis, and is 

 prolonged on to the 

 dorsal surface of the 

 proboscis and the dor- 

 sal surface of the arms. 

 On the ventral side of 

 this nerve-strand is a 

 very slender cylin- 

 drical cellular cord 

 (nch.) continuous be- 

 hind with the epi- 

 thelium of the pha- 

 rynx: this is supposed 

 to represent the diver- 

 ticulum of Balano- 

 glossus, and thus to be homologous with the notochord of the 

 Chordata. A blood-vascular system with heart and cardiac sac like 

 those of the Enteropneusta is present. In some species of Cephalo- 

 discus the sexes are united, in most they are separate. The posterior 

 end of the body is drawn out into a sort of stalk on which the buds 

 are developed (Fig. 721). A pair of ovaries (ov.) lie in the trunk- 

 cavity, and there is a pair of oviducts (ovd.) lined by elongated, 

 pigmented epithelium. The development, which is direct, without a 



FIG. 7^-. Cephalodiscus. Diagram, of longitudinal 

 section, a. anus ; be 1 , cceiome of proboscis ; 6c 2 . 

 coelome of collar ; be 3 , coelome of trunk ; int. intestine ; 

 m. mouth ; nek. supposed notochord ; n. s. nerve- 

 strand : op. operculuni ; ces. oesophagus ; ov. ovary ; 

 ovd. oviduct ; ph. pharynx ; p. p. proboscis-pore ; ps. 

 proboscis ; st. stomach ; stk. stalk. (After Harmer.) 



