576 



ECHINODERMATA HOLOTHUROIDEA 



adult. Respiratory trees absent, and transverse muscles of 

 adjacent interradii continuous, so as to form circular muscles. 

 Otocysts attached to the nerve-ring as in Elasipoda. 



The members of this remarkable order, like those of the pre- 

 ceding one, are burrowers ; but though their feelers are larger, 

 the rest of their anatomy has undergone much more profound 



modification than that experi- 

 enced by the Molpadiida. 

 The loss of the radial canals, 

 which must be practically 

 function less in Molpadiida, 

 is not a great step, but the 

 change in the mode of respira- 

 tion is a greater modification. 

 Eespiration appears to be 

 effected by diffusion through 

 the body -wall, which is always 

 comparatively thin. The cir- 

 culation of the body -cavity 

 fluid is assisted by a number 

 of stalked, ciliated cups placed 

 on the mesenteries near the 

 line of their insertion on the 

 body -wall. In dealing with 

 Asteroidea it was pointed out 

 that the ends of the tube- feet are 

 the only places where numer- 

 ous sense-hairs are to be found, 

 and which, therefore, can be 



Fiq. 263. -Synapta digitata. A, animal called sense- organs. This is 

 viewed from the side, x ^ ; B, anterior true generally throughout 



end, with tentacles extended, x 2. a 



Echmodermata. Now m Syn- 

 aptida, where the tube - feet are lost, the surface of the 

 body has scattered over it little sense-organs consisting of 

 hillocks of ectoderm with an aggregation of sense-cells. These 

 may be regarded as representing the discs of the missing 

 tube-feet. One is involuntarily reminded by the ciliated cups 

 and scattered sense-organs of the ciliated urns and sense- 

 organs of the Sipunculidae, which lead a similar life ; and 

 taking into consideration the general superficial likeness in 



