220 



MANUAL OF ZOOLOGY. 



ary vesicles, just as in the Echinus. In the typical forms there 

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

 over the whole body, or may be restricted to the ventral sur- 

 face. There is also a "sand -canal" which arises from the 

 circular canal, and is terminated by a madreporiform tubercle ; 

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

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

 the ambulacral system is filled, is derived from the perivisceral 

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



The mouth is toothless, is situated anteriorly, and is sur- 

 rounded by a beautiful fringe of branched, retractile tentacles 

 (fig. no), which arise from a ring of calcareous plates, and into 

 which are sent prolongations from 

 the circumoral ring of the ambu- 

 lacral system. These tentacles, ten 

 to twenty in number, are really mo- 

 dified tube-feet, and probably serve 

 in part as respiratory organs. The 

 mouth opens into a pharynx, which 

 conducts to a stomach. The intes- 

 tine is long and convoluted, and usu- 

 ally 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, in many forms, two branched 

 and arborescent tubes, the termina- 

 tions of which are csecal. These 

 run up towards the anterior extrem- 

 ity of the body, and together consti- 

 tute 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 vascular 

 system consists of two main vessels 

 Fig. ^.-Pentactafrondosa, show- one dorsal, and the other ventral 



ing the crown of feathery tenta- Connected With a cirCUm-CeSOplia- 



cles round the mouth and the , ->. i r 



rows of tube-feet. geal ring. Development is in a few 



instances direct ; but in most cases 



there is a metamorphosis, the larva being vermiform, and devoid 

 of a skeleton. The nervous system consists of a cord, surround- 

 ing the gullet, and giving off five branches which run alongside 

 of the radiating ambulacral canals. The sexes are generally, 



