572 



THE INVERTEBRATA 



Class HOLOTHUROIDEA 



Sausage-shaped Echinodermata, without arms ; without recognizable 

 abambulacral area ; usually without external madreporite in the adult ; 

 with the ambulacral grooves covered ; some of the tube feet modified 

 into tentacles around the mouth, and some or all of the rest, if present, 

 provided with suckers; a muscular body wall containing very .small 

 ossicles; no spines; and no pedicellariae. 



The typical form of body of the Holothuroidea is well seen in the 

 members of the widely distributed genus Holothuria (Fig. 432 B), to 

 which the familiar British "cotton spinner" belongs. It is such as 

 would result if in a regular echinoid the ossicles were reduced and the 

 body drawn out in the oro-anal axis, the madreporite with the gonad 



.resp.tf. 



buctf. 



Fig. 429. Echinocardium cor datum. A, From the aboral; B, From the oral 

 side, buctf. buccal tube feet; resp.tf. respiratory tube feet: both contracted. 



which adjoins it being displaced along their interradius to a position 

 not far from the mouth, and the other gonads lost. As has been ex- 

 plained on p. 551, the madreporite usually becomes internal. It is 

 so in Holothuria. 



Owing to the presence in one interradius of the primary madre- 

 porite and the gonad, the body always possesses a rudimentary 

 bilateral symmetry. In many cases, as in Holothuria, this symmetry 

 becomes conspicuous owing to the fact that the animal constantly 

 applies to the ground the three radii of the side opposite to the madre- 

 poric interradius, and this side becomes differentiated as "ventral" 

 from the "dorsal" side which contains the madreporic interradius 

 with the two radii which adjoin it. The differentiation consists in a 



