154 MANUAL OF ZOOLOGY. 



to the ventral surface. There is also a " sand-canal," which 

 arises from the circular canal, and is terminated by a madre- 

 poriform tubercle ; but this, instead of opening on the exterior, 

 hangs down freely in the perivisceral cavity. The fluid, there- 

 fore, with which the ambulacral system is filled, is derived 

 from the perivisceral cavity, and not from the exterior, as is 

 usually the case. 



Fig. 45. Holothuroidea. Thyone fapillosa. (After Forbes.) 



The mouth in Holothuria is situated anteriorly, and is sur- 

 rounded by a beautiful fringe of branched, retractile tentacles 

 (fig. 45), which arise from a ring of calcareous plates, and into 

 which are sent prolongations from the circum-oral ring of the 

 ambulacral system. The mouth opens into a pharynx, which 

 conducts to a stomach. The intestine is long and convoluted, 

 and opens into a terminal dilatation, termed the "cloaca," 

 which serves both as an anus and as an aperture for the 

 admission of sea-water to the respiratory tubes. From the 

 " cloaca " arise two branched and arborescent tubes, the 

 terminations of which are probably caecal. These run up 

 towards the anterior extremity of the body, and together 

 constitute the so-called "respiratory tree." They are highly 

 contractile, and they perform the function of respiratory organs, 

 sea-water being admitted to them from the cloaca. The 

 nervous system consists of a cord, surrounding the gullet, 

 and giving off five branches, which run alongside of the radi- 

 ating ambulacral canals. The generative organs are in the 

 form of long, ramified, caecal tubes, which open externally by 

 a common aperture, situated near the mouth. There is thus 

 no trace of that radial symmetry which is observed in the 

 arrangement of the reproductive organs in the other orders of 

 the Echinodermata. The vascular system consists of two main 

 vessels one dorsal, and the other ventral connected with a 

 circum-cesophageal ring. 



The skin in the Holothuria is highly contractile, and the 

 body is provided with powerful longitudinal and circular 

 muscles, in compensation for the absence of .any rigid integu- 



