Introduction to Animal Morphology. 235 



laria, but in others it is at the bottom of an atrial or 

 cloacal chamber beside the branchial cavity, whose 

 opening may be simple or surrounded by a 10-11- 

 cleft rosette. In the simple forms the cloaca is an ir- 

 regular sac lined by a thin membrane. The neural 

 side of the branchial sac projects into this chamber, 

 and the partition is pierced by many openings with 

 ciliated borders, whereby the water from the branchial 

 enters the atrial chamber. The atrial opening in 

 fixed forms is usually anterior and dorsal, close to the 

 mouth. In free swimming forms it is usually directed 

 backwards. In Polyclinidae a tongue-like process 

 can close this opening, and in others its lips may be 

 crenated. In colonial forms each cluster of personae 

 may be grouped radially around a common cloaca, 

 into which their intestines open. 



The nervous system consists of a supra-pharyngeal 



lion placed usually between the oral and cloacal 

 openings. A pair of nerves loop around the mouth, 

 and may form a circumpharyngeal ring, but without 

 an inferior ganglion. The nerve threads are primitive 

 nerve fibrils, and those distributed to the muscles 

 appear continuous with the fibres thereof ; branches 



to the sphincters, to the muscular lamina, and to 

 the viscera. In Appendicularia a nerve extends 



h'' tail, in which it shows several successive 



(This tail in Appcndirularia acts as a .swimming 

 , is lonLT'T than thr body, and consists of a la\T 

 :nis with or without epithelium, a 1 i\ <T <f cir- 

 cular, ,'iii'l on*- of longitudinal fibn-s with tin- nerve 



<-'lK in a homo- 

 gen' | tli- tail 



