478 COMPARATIVE ANATOMY CHAP. 



manner above described, and thus runs in a spiral ; (3) in most cases, 

 close examination reveals that even in the adult the canal is coiled in 

 a spiral, although drawn out to a great length, and that the first and 

 second bendings can still be distinguished as slight curves. 



If the typical alimentary canal of an Adinopod were to be 

 shortened until it was of almost the same length as the body, the con- 

 dition found in the Synaptidce would arise. 



The divisions OP sections of the alimentary canal. In the 

 intestine of the Holothurioidea, consecutive sections have been 

 distinguished, but these are never very marked microscopically. 

 Throughout its whole course, the canal retains its tubular shape. 

 The different sections are distinguished by their sizes and by the 

 thickness of their walls, by their colour, their vascularisation, and 

 especially by their histological structure. The boundaries of the con- 

 secutive sections are usually externally indicated by circular constric- 

 tions of varying distinctness; these constrictions not infrequently 

 correspond with circular folds projecting into the canal. 



The mouth. Around the mouth, the circular musculature becomes thickened into 

 a small sphincter muscle. 



The more strongly the oral tentacles are developed, the more marked is the 

 capacity for invaginating the mouth with its tentacles, and with a larger or smaller 

 portion of the anterior end of the body, into the body cavity. In the Dendrochirotcc, 

 in which the tentacles are strongly developed, the invaginable portion of the anterior 

 end of the body is called the proboscis. It is not infrequently distinguished by the 

 different colouring and constitution of its integument. In all cases, in invagination, 

 the chief part is played by the retractor muscles (cf. p. 471). At the posterior 

 boundary of the proboscidal region five (interradial or radial) calcareous valves are 

 occasionally developed ; these, when the proboscis is invaginated, close the aper- 

 ture (e.g. Psolus, Figs. 227 and 228, p. 287). 



The oesophagus reaches from the mouth to the circular canal of the water 

 vascular system, or even further. It is attached to the water vascular ring, the cal- 

 careous ring, the radial canals of the water vascular system, etc. , by means of bands 

 which run out radially, traversing the pericesophageal sinus (see Fig. 365, p. 428). 

 These bands are chiefly of the nature of connective tissue, but also contain muscle 

 fibres. The oesophagus, with the complex of surrounding organs, is sometimes 

 called the pharyngeal bulb. 



The oesophagus is followed by a shorter portion known as the stomach intestine, 

 and this again by the longest part of the digestive tract, the small intestine. This 

 last forms the larger posterior portion of the first section of the intestine, the whole 

 of the second section, and by far the greater portion of the third and last section. 



The last part of the alimentary canal, the cloaca or rectum, is distinguished by 

 special thickness, and is attached by radially arranged strands and filaments to 

 the neighbouring body wall. These strands consist of connective tissue and muscle 

 fibres. 



Into the cloaca or rectum open the water lungs and the Cuvierian organs, 

 where these are present. These will be described, pp. 487, 488. In some Elpidiidce, 

 the anterior part of the cloaca bulges out to form a large caecum, which projects more or 

 less far into the body cavity, sometimes reaching almost to the middle of the body. 

 Since the Elpidiidce possesses no water lungs, there is some justification for the 



