ECHINODERMA. 117 



constituent plates of the test are the more firmly united. (For details, 

 see Prof. Martin Duncan, in Journal Linnean Society, vol. xvi. 

 p. 348y 



Some, for protection, when exposed to great waves live in hollows 

 of the rocks, and a fine photograph shows Purple Sea-Urchins 

 {Strongylocentrotus lividus) (.596) in hollows made by them in lime- 

 stone rocks, Bundoran, South Donegal. 



The Irregular Echinoids (613-711) are distinguished from the 

 regular forms by never having the vent at the pole of the body 

 opposite to the mouth, but posterior to it ; it is also interradial in 

 position. 



The mastigatory apparatus (Lantern of Aristotle) is reduced or 

 lost ; in the former case the Urchins are known as G-nathostomata, 

 and in the latter as Nodostomata. 



The Gnathostomata (613-710) are largely fossil ; they have a 

 central mouth with teeth and jaws, the ambulacra are simple or 

 petal-like, and are all similar. 



The NoDOSTOMATA (652-711) have the mouth either central or 

 pushed forwards, and there are no teeth or jaws. The circular form 

 of the regular Echinoid is often hidden by a bilateral symmetry 

 produced by a special modification of the anterior ambulacrum. 



The genus Hemiaster (707) 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 (713). 



The HoLOTHURioiDEA, 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 lY. (150-188). According 

 as they have or have not the sucking-feet of the Echinoderma, they 



