138 Echinoderma. 



The peristomej ia most of the regular forms, bears ten dendritic 

 outgrowths of the body-wall, the oral branchiae, close to the edge of 

 the shell ; in others, however, they are wanting. 



On the ventral side of the shell, close to the peristome, there are attached to 

 the pore-plates of most Echinoids tiny globular structures, with short stalks and 

 a glassy, calcareous skeleton. These so-called spheridia are probably sense- 

 organs, possibly gustatory or olfactory. 



Urchins occur in all seas, and are abundant both in genera 

 and species. They were also well represented in earlier geological 

 periods. 



Order 1. Echinoidea regularia {Eegular 



Sea-urchins). 



The anal area lies at the upper pole. The body is usually 

 approximately spherical. Spines strong. Teeth present. Dermal 

 branchiae usually present. 



The Regular Sea-urchins feed partly upon other animals, e.g., large 

 Crustacea, which they catch by meaiis of their tube-feet ; partly upon 

 the Polyzoan and Hydroid colonies attached to them, and also to a 

 certain extent upon Algae. Some of them use their teeth to form 

 small cavities in the rocks in which they live. 



As examples may be given : Cidaris, with long strong spines, without dermal 

 branohise ; Echinus, with smaller spines, to which Toxopneustes (Rg. 89) is 

 closely aUied; Echinomeira, with an oval shell. The genus Asthenosoma 

 differs from other Urchins, in that the skeletal plates, which are imbricate, are 

 movably connected. 



Order 2. EcMnoidea irregularia {Irregular 

 Sea-tircldns). 



The anal area has moved into an inter-radius. The body 

 is round or, more often, ovate. Spines small, often bristle-like. 

 Usually toothless. Dermal branchiae absent. 



1. Shield-urchins or Clypeastridse (senus Clypeaster, and others) 

 differ from other Irregular Sea-urchins in the possession of teeth. Shell thick ; 

 mouth in the centre of the ventral surface. Rarely found in Em-opean seas. 



2. Heart-urchins or Spatangidse (genus Spatangus, etc.), toothless, 

 shell usually thin, mouth moved forwards. Peed by ingesting material from, the 

 bottom of the sea. Several species in the North Sea, among them Brissopsis 

 lyrifera (Fig. 90). 



Class 4. Holothuroidea {Sea- Cucumbers). 



In the Holothurians the principal, is always longer than the lateral^ 

 axis, usually several times as long, so that the body is cucumber-, 

 sausage-, or worm-shaped. In correlation with this, the Holothurians 



