ECHINODERMA. 115 
The genus Hemiaster offers an example of an Echinoderm in 
which the eggs are laid in special pouches ; the hinder ambulacra are 
deepened to form pits, which are guarded by specially elongated 
spines (see Case 34) ; in these pits the young pass through all the 
stages of their development. 
The minute structure of the spines of Sea-Urchins is illustrated 
by a series of figures on the wall. 
The Holothttrioidea, or Sea-Cucumbers, form the last order of 
Echinoderms. Their body, as indicated by their English name, is 
elongate, subcylindrical, with a more or less flexible integument, 
according to the extent of the reduction of the calcareous skeleton ; 
the mouth is at one end of the body and surrounded by tentacles, 
the vent at the opposite end. 
As these animals cannot be shown in a dried state, some of them, 
preserved in spirit, are placed in Wall-Case IV. According as they 
have or have not the sucking-feet of the Echinoderma, they are 
ordinarily divided into the Pedata and the Apoda ; the latter are 
represented by Synapta, which may attain to a great length, and 
by Chiridota ; the Pedata are illustrated by the genera Cucumaria, 
Psoitis, and Holothuria. Deep-sea investigations have revealed the 
existence of another group of specially modified Holothurians — the 
Elasipoda ; these are remarkable for their well-marked bilateral 
symmetry and the distinctness between the dorsal and ventral 
portions of the body ; the prominent processes on the dorsal surface 
are not contractile. 
An exhibition of some interest is to be found in a Table-Case 
against the wall, in which there are various specimens of the edible 
Holothurians — trepang or beche-de-mer ; these were all bought in the 
market at Canton, and may be taken to be typical of the kinds 
offered for sale in various eastern countries. 
