HOLOTHURJANS. 



177 



d- 



V 









There is no dental apparatus in the holothurids, but a short pharynx leads into a 

 stomach, and this into an intestine usually much longer than the body, and often 

 several times its length. In the 

 higher holothurids the intestine 

 terminates in a distinct chamber 

 or cloaca, often of large size. Into 

 this cloaca open the stems of two 

 (sometimes one) hollow, much- 

 branched organs called the 'res- 

 piratory trees.' The sea-water 

 enters into and is expelled out 

 of these organs, which thus fill 

 the body-cavity with water that 

 is taken up by the madreporic 

 body and carried into the water- 

 system. The respiratory trees 

 are also believed to be organs of 

 excretion. 



In some holothurians the 

 cloaca or the respiratory tree is 

 also provided with simple or 

 branched appendages the interior 

 of which is occupied by a solid 

 or viscid substance. The use of 

 these is not certainly known, but, 

 as they are readily thrown out 

 when the animal is disturbed, 

 Semper supposes that they are 

 organs of defence. These are 

 known as the Cuvierian organs. 



The nervous system is of the 

 usual echinoderm pattern, consist- 

 ing of a mouth-ring placed above 

 the ring-canal of the water-system, 

 and of five principal ambulacral 

 cords passing through notches in 

 the plates around the cesophagus. 

 The system which is supposed to 

 be analogous to the circulatory 

 system of higher animals is very 

 complex in many of the higher 

 holothurids, extends over the ali- 

 mentary canal, and enmeshes one 

 of the respiratory trees. 



The genital organ is in many 

 cases single, and in the Synap- 

 tidae contains both ova and sper- 

 matozoa, so that these forms are 



VOL. I. —12 



Fig. 156. — Anatomy of Caudina arenata ; a, anastomoses of dorsal 

 blood-vessels; 6, branchial tree; rf, dorsal blood-vessel; /, mesen- 

 terial filaments; ff, genital opening; i, alimentary canal; i, longi- 

 tudinal muscles; m, mouth; 0, genital duct; p, pharyngeal ring; 

 r, reproductive organs, cut away on right side; ^, tentacular am- 

 pullae; V, ventral blood-veesel. 



