Class 4. Holothuroidea. 



139 



do not rest on one pole as do other EclimodermSj but upon one side of 

 the body ; and, as a consequence, one side is often specially developed 

 or even flattened (Fig. 93), so that external radial symmetry is more 

 or less destroyed ; the side turned downwards is termed ventral, 

 the other "dorsal. 



Pig. 93. 



Fig. 92. Diagram of a. Holotliurian; the body- wall is out through and spread 

 Out. a anus, c Cuvierian organs, g gonad, k ring canal, Icp calcareous ring, I respiratory 



tree, ma madreporite, j; polian vesicle, r radial water ressel, t gut, te tentacles. Modified 



from Ludwig. 



Fig. 93. Transverse section of the body-wall of a Holothurian, diagrammatic, a 

 Radial water vessel, I longitudinal muscle, n radial nerve (the white spot above n is the 

 radial blood vessel), t transverse muscle, v body-wall. — After Ludwig. 



Fig. 94. Transverse section through a radius of the body- wall of a Holothurian. 



o/p ampulla, 6 radial blood vessel, s tube-foot ; other letters as in the preceding figure. 



After Ludwig. 



Another feature which is very characteristic of the Holothurians 

 is the softness of the body-wall ; the wall is indeed, as in other 

 Bchinoderms, provided with calcifications, but to so slight an extent 

 as to render it impossible to speak of a dermal skeleton. The calcifi- 

 cations are usually in the form of minute, often microscopic, particles 



