108 PHYLUM ENTEROPNEUSTA. 



and is usually entirely hidden by the ventral portion of the over- 

 hanging proboscis. 



The alimentary canal (Fig. 78) consists of mouth-cavity, 

 pharynx, oesophagus, stomach, and intestine which opens by the 

 posterior and dorsal anus. The bulk of it is contained in the 

 great ventral hump so characteristic of this animal. 



The anterior end of the pharynx gives off a forward diverti- 

 culum which extends through the collar region into the proboscis : 

 this is the notoehord (Fig. 78, 21). There are two gill- slits, one 

 on each side leading outward from the pharynx and opening 

 ventro-laterally on the anterior part of the trunk. The body- 

 cavity is arranged as in other Enter opneusta, i.e. it consists of an 

 unpaired chamber in the proboscis which opens to the exterior 

 by a pair of pores at the junction of the proboscis and collar 

 region of the body (Fig. 78, 20) ; of a pair of chambers in the 

 collar region, which also open outwards by a pair of collar pores 

 (Fig. 79, 9), and of another pair in the trunk which are in rela- 

 tion with the greater part of the alimentary canal and with the 

 ovaries, and which do not open to the exterior. The two halves 

 of the body-cavity of the collar overlap dorsally the hinder 

 part of the proboscis region (Fig. 78, j). The dorsal and ventral 

 mesenteries appear to persist, completely or incompletely, in the 

 collar and trunk regions. The sexes are separate and all the 

 individuals of one colony are of the same sex (except in C. nig- 

 rescens, p. 109). The ovaries are paired sacs placed dorsally in 

 the anterior part of the trunk region. They open to the exterior 

 by two short oviducts, the walls of which are richly pigmented 

 (Fig. 78, ij}. The male colonies appear, in some species (e.g. 

 sibogae), to be dimorphic, containing male individuals which are 

 characterized by possessing two arms without tentacles, a long 

 pedicle and an abortive alimentary canal, and neuter individuals 

 which are like the females but without ovaries. The coelom of 

 the proboscis and collar are to some extent obliterated by muscle 

 and connective tissue, as is the prolongation of the trunk body- 

 cavity in the pedicle. The organs appear to be separated by a 

 basement membrane as in the Balanoglossida and there is no 

 connective tissue of the usual kind except in. the coelomic sacs in 

 the dorsal region of the collar. The central nervous system 

 is contained in the ectoderm. It extends forwards on to the 

 hinder part of the proboscis and the proboscis pores perforate its 



