ECHINODERMA TA. 



177 



external brood-poucli {marsupium) , in which the eggs 

 are hatched ; and, as happens among those star- 

 fishes which are viviparous, in some of these forms 



Fig. 45. — Platens larvse (outline) of an Ecliinus,- E. Tnicrotuherculatus : m, 

 mouth. (Magnified.) 



the young skip the larval stage and develop directly 

 from the egg. 



The HoLOTHUEOiDEA, or Sea-cucumbers, are animals 

 worm-like in appearance, in which the characteristic 

 dermal skeleton of the Echinodermata is reduced to 

 scattered spicules in a leathery body- wall. They have 

 tentacles which belong to the water-vascular system ; 

 but the tube-feet tend to degenerate, and in some 

 forms are entirely wanting. Their symmetry tends 

 towards the bilateral type, although the tentacles are 

 radially arranged round the mouth; the body is 

 elongated. They are often nocturnal and predatory, 

 and sometimes phosphorescent. The common Sea- 

 cucumber is their representative in English seas. 



