ANNULOIDA : ECHINODERMATA. 179 



ambulacral system is filled, is derived from the perivisceral 

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



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

 rounded by a beautiful fringe of branched, retractile tentacles 

 (fig. 67), 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 termina- 

 tions 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 



Fig. 67. Holothuroidea. Thyone papillosa. (After Forbes.) 



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, caecal 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 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- 

 mentary skeleton. Many of the Sea-cucumbers, in fact, are 

 endowed with such high contractility that they can eject their 

 internal organs entirely, if injured or alarmed. 



In Synapta there is no ambulacral system of tube-feet, nor 

 respiratory tree. Locomotion is effected by means of little. 



