10 



ZOOLOGY 



SECT. 



not in organic continuity, so that, though enclosed in a common 

 investment, they do not form a colony in the sense in which the 

 word is used of the Polyzoa or the Hydroid Zoophytes. They have 

 the 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 with numerous very fine 



pinnately -arranged tentacles 

 and containing a prolongation 

 of the collar-cavity. The pro- 

 boscis (Fig. 713, ps.) is a 

 shield -shaped lobe overhang- 

 ing the mouth ; its cavity 

 communicates with the ex- 

 terior 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 re- 

 gion on each side is a small 

 area in which the body-wall 

 and that of the pharynx are 

 coalescent ; this area is per- 

 forated by an opening the 

 gill-slit. Cilia occur only on 

 the arms, proboscis and lateral 

 lips. A nerve- strand contain- 

 ing nerve-fibres and ganglion- 

 cells is situated on the dorsal 

 side of the collar, and is pro- 

 longed on to the dorsal surface 

 of the proboscis and the 

 dorsal surface of the arms. 

 On the ventral side of this 

 nerve-strand is a very slender 

 cylindrical cellular cord (nch.) 

 continuous behind with the 

 epithelium of the pharynx : 

 this is supposed to represent 



the diverticulum of Balanoglossus, 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. The nervous system lies deeper than the epidermis ; 

 it comprises a dorsal ganglion (collar-cord) situated in the 

 collar. In some species of Cephalodiscus the sexes are united, 

 in most they are separate. The posterior end of the body is 



FIG. 711. Cephalodiscus. Gelatinous 

 investment. (After Mclntosh.) 



