ORDER ECHINODERA. 185 



thickened beneath the interval between two successive rings ; 

 it consists of a granular layer of protoplasm in which scat- 

 tered nuclei occur. Beneath the ectoderm lies a somewhat 

 incomplete layer of longitudinal muscles, which become spe- ' 

 cialized anteriorly into separate bundles for the retraction of 

 the proboscis; in each metamere two dorso-ventral muscle- 

 bundles, one on each side of the middle line, are also found. 

 A relatively spacious body-cavity in which various organs 

 lie occurs, but no lining peritoneal epithelium or mesenteries 

 have been observed. 



The digestive tract begins with the mouth, which lies at 

 the bottom of the invaginated proboscis and opens by the in- 

 tervention of a short tube into a muscular pharynx into the 

 anterior portion of which, four glands, either salivary or poi- 

 sonous in function, pour their secretion. The pharynx com- 

 municates posteriorly with a sac-like stomach, upon which 

 follows a short straight intestine opening to the exterior at 

 the posterior end of the body between the terminal cerci. 



Two elongated pear-shaped bodies lying in the ccelom in 

 about the middle region of the body have been described as 

 excretory organs. They are closed at the free end, their 

 cavity is ciliated, and they open to the exterior on the dorsal 

 surface near the margin of the body. The reproductive 

 organs are cylindrical sacs which are provided with ducts 

 opening to the exterior on the terminal segment ; all the 

 species whose reproductive organs have been studied are 

 bisexual. 



Four cellular masses lying above the pharynx seem to 

 represent the nervous system, though no nerves passing from 

 them have been discovered ; nor do any special sense-organs 

 exist. 



The affinities of these forms is highly problematical, especially since 

 nothing is known of their development. The metamerism indicated by the 

 chitinous rings, the thickenings of the ectoderm, and the dorso-ventral 

 muscles suggest an affinity with the Annelids, while, on the other hand, in 

 the chitinous covering, and the occurrence of a longitudinal musculature 

 only, similarities to the Nematodes may be found. The excretory organs 

 may perhaps be compared with the larval nephridia of the Annelids, and 

 the existence of but a single pair of them, together with the absence of 

 any metameric arrangement of nerve-ganglia, favors the idea that the 



