204 MORPHOLOGY OF INVERTEBRATE TYPES 



consider the radius to which they belong, that is, the median 

 ventral radius of the trivium, as the fifth radius; the adjoining 

 left ventral radius as the fourth radius, and the right ventral 

 radius as the first radius; the right bivial radius as the second 

 and the left bivial radius as the third radius. Behind the tenta- 

 cles is a short collar strengthened on the inside by a calcareous 

 peripharyngeal ring or corona consisting of five radial and five 

 interradial plates. The ambulacral tubes of the ventral surface 

 are numerous and developed as true ambulacral feet with ter- 

 minal suckers; those of the bivium or dorsal surface are less 

 numerous and devoid of suckers, i. e., they are modified into 

 ambulacral t&ntacles. Terminal tentacles are absent. 



The body wall is devoid of an articulated calcareous skeleton. 

 Instead it is soft and muscular with irregular perforated calca- 

 reous plates in its dermal layer, mainly in the anterior and pos- 

 terior regions of the body. Perforated calcareous plates are 

 found also in the wall of the tentacles and of the suckers of the 

 feet. The external body covering consists of an epidermis which 

 is not ciliated. Under the dermal layer is a heavy sheet of 

 circular muscles. Five longitudinal muscles each consisting of two 

 bundles run the whole length of the radii. The pharynx has five 

 strong retractors, each consisting of two bundles, and the cloaca 

 has numerous dilating muscles dilatatores cloacce which run 

 radially in all directions from the cloaca to the body wall. The 

 body cavity is lined with a ciliated peritoneal epithelium. 



Digestive system. The mouth situated at the anterior end 

 of the body is surrounded by a circular lip but devoid of teeth 

 or jaws. The pharynx is a cylindrical tube provided with five 

 retractors already mentioned. Between the pharynx and the 

 short oesophagus is a constriction. The stomach is small but 

 muscular. The intestine is thin, very long and convoluted, runs 

 to the posterior end, turns back to the anterior end and turning 

 again runs to the posterior end where it forms a short rectum 

 and opens into the cloaca. The anus or opening of the cloaca 

 to the outside is in the centre of the aboral end of the animal, 



