LOWER INVERTEBRATES WHICH BREATHE IN WATER 413 



general surface of the body, the limitation being greatest where 

 the skeleton is best developed and most continuous. 



Crinoids. — In a Feather-Star {Comatuld) the body consists of 

 a central cup, from the margins of which five branching feathery 

 arms grow out. One side of the cup and the corresponding sides 

 of the arms are strengthened by a continuous set of calcareous 

 plates, through which breathing cannot take place. But the other 

 side of the body, in the centre of which the mouth is situated, is 

 covered by leathery skin, and it is necessarily this side which does 

 the work of respiration, chiefly by means of special outgrowths, as 

 will be explained later on. In Sea- Lilies it is usual for both sides 

 of the body to be supported by firm plates, between which the soft 

 breathing organs project. 



In Sea-Urchins (Echinoids) the body is supported and pro- 

 tected by a firm test, composed of numerous calcareous plates 

 united by their edges, and thus the area of the general surface 

 available for breathing is very much reduced. The mouth, how^ 

 ever, is situated in the middle of a soft membrane which no doubt 

 partly performs this function, and its efficiency is greatly increased 

 by the presence of 

 five pairs of branched 

 oral gills which pro- 

 ject from it (fig. 542). 

 The digestive tube is 

 also specialized to 

 assist in respiration, 

 for part of the water 

 which enters the 

 mouth with food is 

 conducted along a 

 narrow tube ("si- 

 phon") that branches 

 off from the gullet, 

 and later on opens 

 into the intestine. 



In ordinary Star- 

 Fishes (Asteroids) the skeleton is not so complete as in the last 

 group, so far as the upper surface and the sides are concerned, 

 and numerous thin plates are left, from which small branching 

 gills are protruded (fig. 543). 



^'^Z- 543' — Cross section through Arm of Star-Fish enlarged 



g. Gill; PPPP, calcareous plates (hard parts are shaded obliquely). Two 



tube-feet are seen below 



