246 



ZOOLOGY 



and contains small scattered calcareous ossicles. The mouth 

 is surrounded hy a circlet of retractile tentacles, into which the 

 water-vascular ring sends extensions. The madreporic plate 

 usvMlly opens into the hody -cavity. Tlie anus is usually 

 terminal. 



The body of the Holothurians is elongated along an oral- 

 apical axis. The ambulacra are five in number ; they may be 

 equally developed, or three of them, 

 the trivium, may be flattened and 

 form a creeping sole upon which the 

 animal rests ; the bivium is then con- 

 vex. This occurs in Psolus and in all 

 the Elasipoda. When this specialisa- 

 tion of radii takes place, the tube-feet 

 are modified on the trivium. In other 

 ises the tube-feet are scattered all 

 rer the body, and in others — the 

 ynaptae — they are entirely wanting. 

 I he skin is covered by an ectoderm 

 ith an external cuticle ; within this 

 a layer of connective tissue, in which 

 ills laden with pigment and cal- 

 ireous ossicles are scattered. This 

 I yer also includes a nervous plexus. 

 The connective tissue sheath surrounds 

 a muscular layer whose fibres run in 

 a circular direction, and more inter- 

 nally are five radial bands of longi- 

 tudinal muscles, one running along each 

 ambulacrum, and lying beneath the water-vascular vessel and 

 nerve ; anteriorly these bands are attached to the pharyngeal 

 ossicles, which are radial and interradial in position. The 

 ossicles in the integument are always small in size ; they may 

 be simple spicules, or may assume a number of very elegant 

 forms in the different genera. In the Elasipoda they exist 

 in the mesenteries and in the walls of the alimentary canal, 

 as well as in the integument. 



The coelom is large, and is lined with ciliated cells ; a 

 special section of it surrounds the pharynx, and in the outer 



Pig. 148. — Holothuria 

 papulosa. 



