Characteristics of Holothurioidea. cxlvii 



may be allowable to suggest that we have in reality a case of intra-marsu- 

 pial development, such as that known to occur in Fteraster militaris. 



The radial character of the future Echinoderm is first sliowu in 

 the formation of the ambidacral elements of the water- vascular 

 system, which first shows itself as a simple tubular depression, the 

 future 'madreporic^ system, on the dorsal surface of the larva. 

 The Echinodermata are exclusively marine ; some of the Holothu- 

 rioidea, however, can live in brackish water, or, rather, in the sand 

 and mud of estuaries. They are never social ; and, with the 

 exception of Pentacrinus, all the living* representatives of this sub- 

 kingdom are free. 



Class, Holothurioidea. 



Echinodermata, varying in shape from being subcylindrical and 

 vermiform to being plano-convex like an ordinary snail, with an in- 

 tegument different from that of other members of the sub-kingdom, 

 with the exception of a single Echinus, in being, through default 

 of development of the calcareous skeleton, left supple and pliable. 

 Their mouth is surrounded by a circlet of tentacles which are modi- 

 fications of the ambulacral feet ; are frequently used as locomotor 

 organs ; and in certain Synaptidae^ as also in the developing stages 

 of other Holothurioidea, are provided with suckers. With a few 

 exceptions {Spmptidae and Molpadidae), the Holothurioidea have a 

 well-developed radial ambulacral system, in addition to their peri- 

 stomial ambulacral tentacles. If the ambulacra are arranged in five 

 functionally similar rows, the body of the animal obtains a somewhat 

 pentagonal outline ; if they are scattered over the entire surface so as 

 to leave no distinct inter-ambulacral areae, the body is vermiform in 

 general outline ; if the five rows are divided into a ventral locomotor 

 trivium ; and a dorsal bivium, the ambulacral feet in which may have 

 no suckers, or be wholly absent ; the body comes to resemble that of 

 a Gasteropodous Mollusc. In the Molpadidae the radial stems of 

 the water-vascular system are present, as also certain tubular pro- 

 longations of them which pierce the skin, but they have no feet 

 developed upon them ; in the Synaptidae the water- vascular system 

 has no radial stems, and consists simply of the circum-oral ring", 

 with its Polian and madreporic appendages, and its ampullae and 

 branches for the tentacles. In some cases the Polian, glandular, 

 and madreporic appendages of the water-vascular ring are very 



k 2 



