10 



ZOOLr)GY 



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 

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

 usually eight to sixteen arms, each beset with numerous very fine 



piinnately -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- 

 joores. 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 

 (jill-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 surfeee of the arms. 

 On the ventral side of- this 

 nerve-strand is a very slender 

 cylindrical cellular cord (««/(.) 

 continuous behind with the 

 epithelium of the pharynx : 

 this is supposed to represent 

 diverticulum of Balanoglossus, and thus to be homologous 

 with the notochord of the Cliordata. A Vilood -vascular system 

 with Jieart and cardiac sac like those of the Enteropneusta 

 IS present. The nerv(jus system lies deeper than the epidermis ; 

 it Cdiiiprises a doi'sal ganglion (colJar-coiTl) situated in the 

 collar. In some species (if C(]ihalo(liscus the sexes are united, 

 m most they are separate. The jiosterior end of tiie body is 



Vie. 711.— CepUalodiscuB. Gelatinous 



investment. (After Melntush.) 



thL 



