TYPE PROTOCHORDATA. 599 



the mouth. Behind these is what is termed the collar region 

 (F), which bears upon its dorsal surface two long armlike 

 processes ( Go), each carrying a double row of ciliated tentacles 

 (G) arranged pinnately. The third region is the visceral sac 

 (), from the posterior and ventral portion of which the stalk 

 arises, while dorsally and anteriorly it carries a stout papilla 

 (5), at the extremity of which the anus opens. 



The digestive tract consists of a straight oesophagus trav- 

 ersing the collar, and having connected with it on the dorsal 

 surface a short blind process (N) whose cavity communicates 

 with that of the oesophagus. This is the rudimentary noto- 

 chord. The oesophagus opens into a large saclike stomach, 

 from the lower end of which the intestine (7) arises, and, bend- 

 ing upon itself, runs forwards to open on the anal papilla. 



The nervous system consists of a thickening of the ecto- 

 derm on the dorsal surface of the collar region, where is also 

 found a small ciliated elevation (Ii) supposed to be sensory ; 

 no other special sense-organs occur. On each side of the 

 collar a pore occurs which, by a short ciliated canal which 

 perforates the wall of the body, places the coelom of the collar 

 in communication with the exterior, and may be regarded as 

 representing an excretory organ. No epistome-pore or branchial 

 slits have yet been observed. 



Cephalodiscus resembles Rliabdopleura in being colonial, but 

 the house is gelatinous, and the various buds formed from the 

 short stalk do not remain in connection with each other, but 

 early separate from the parent. Each polyp (Fig. 273) con- 

 sists of three regions an anterior prseoral portion which forms 

 a large epistonie, a middle collar region, and a posterior visceral 

 sac ; the body-cavity being divided into three corresponding 

 regions. Two epistome-pores occur, the canal leading from 

 the ccelom to them passing through the anterior part of the 

 nervous system (n). The dorsal ectoderm of the collar region 

 is thickened to form the central nervous system, and on each 

 side of this is a cluster of six tentacles (t), each ending in a 

 kuoblike dilatation and bearing numerous lateral pinnules 

 arranged in two rows. At the sides the collar is continued 

 backwards as a pair of lateral folds which slightly overlap the 

 anterior portion of the visceral sac and form the operculum, 



