146 ECHINODERMATA. 



nera ; our knowledge of those singular animals being entirely derived 

 from the exterior conformation of two recent species, and from the 

 mutilated skeletons of fossil Encrinites, which exist in such abun- 

 dance in the limestone strata of our own country. 



Commencing, therefore, with the Asteridce,* we shall now enter at 

 once upon the consideration of the anatomy of such species as have 

 been most carefully examined, and merely notice incidentally the 

 modifications which occur in the disposition of various organs in 

 kindred genera. 



(188.) On examining a living Asterias, the outer covering of its 

 body is found to be composed of a dense coriaceous substance, in which 

 numerous calcareous pieces are apparently imbedded. The cori- 

 aceous integument is generally coloured externally with lively tints, 

 and is evidently possessed of considerable irritability, as it readily 

 shrinks under the knife, or upon the application of various stimuli. 

 When cut into, it has a semicartilaginous hardness, and fibrous 

 bands, almost resembling tendon in their aspect, may be seen to 

 radiate from the centre of the body towards the extremities of the 

 rays. There is no doubt that the movements of the rays are 

 effected by the contractions of this fibrous membrane ; and that, 

 especially in the most polyp-like forms, as in Comatula and Gor- 

 gonocephalus, the irritable skin is the principal agent in effecting 

 locomotion. 



Besides the calcareous matter deposited in its interior, this outer 

 covering of the star- fish appears to furnish several secretions of 

 different descriptions. The colouring matter upon its surface is no 

 doubt one of these ; as is a reddish fluid which exudes from the in- 

 tegument of A. rubens, and is of so caustic a quality as occasion- 

 ally to produce great irritation of the skin in persons by whom 

 individuals of this species are incautiously handled : moreover, in 

 A. aranciaca, the whole animal is coated with a thick mucus, so 

 dense and filamentous that it may be raised in thin films resembling 

 a cobweb, and might easily be taken for a cuticular covering. 



The exterior of the body is generally rendered rough and un- 

 even by various structures, either imbedded in the substance of 

 the coriaceous skin or projecting from its external surface. We 

 have already described the articulated pieces attached to the rays of 

 Comatula and others, which seem to be the most perfectly de- 

 veloped forms of these cutaneous appendages. In the common 

 star-fish of our own coast, similar spinous processes, but composed 



* The name of this family, and of its typical genus, is derived from ao-rng, a star. 



