ANNULOIDA: ECHINODERMATA. l6l 



vesicks, just as in the Echinus. In the typical forms there 

 are five rows of tube-feet, but these may be scattered over the 

 whole body, or may be restricted to the ventral surface. 

 There is also a " sand-canal," which arises from the circular 

 canal, and is terminated by a madreporiforra tubercle; but 

 this, instead of opening on the exterior, hangs down freely in 

 the perivisceral cavity. The fluid, therefore, with which the 

 ambulaoral system is filled, is derived from the perivisceral 

 cavity, and not from the exterior, as is usually the case. 



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

 rounded by a beautiful fringe of branched, retractile tentacles 

 (fig. 47), 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 



Fig. 47. — Holothuroidea. Tkyone papulosa, (After Forbes.) 



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

 cloaca arise two branched and arborescent tubes, the termina- 

 tions of which are probably csecal. 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 radiating ambulacral 

 canals. The generative organs are in the form of long, rami- 

 fied, csecal tubes, which open externally by a common aper- 

 ture, 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 EMnodefmata. 

 The vascular system consists of. two main vessels — one dorsal, 

 and the other ventral — conn,ected with a circum-oesophageal 

 ring. 



