296 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



arranged slits ; the edges of these slits often bear 

 small spines, while the oral faces of the ossicles carry 

 similar spines, the so-called oral papillae ; this 

 armature of fine spines serves no doubt as a filtering 

 apparatus to the digestive cavity of these aproctous 

 Echinoderms. We may correlate the injuries which 

 the arms are often seen to undergo with the absence 

 of defensive spines ; the Ophiuroid leaves an arm with 

 the foe from which it is unable to defend itself ; and 

 we may compare with this the arrangements of the 

 tail vertebrae of the harmless lizard. (See page 322.) 

 Arms thus broken are in time renewed. While in 

 the Ophiurida the arms are nearly always un- 

 divided, however long they may be, the Astrophytida 

 exhibit various stages of division ending in the great 

 complexity of the free termination of the arms which 

 obtain in the basket-fish, or gorgon's head (Astro- 

 phyton, Gorgonocephalus) ; in the Astrophytida the 

 spines are reduced to a minimum, and the integument 

 is thick and leathery. 



Though many Holothiirians have a very thick 

 skin, and a deposit only of spicules in their integu- 

 ment, we cannot suppose that this is a retention of 

 the primitive condition, spoken to by the fact that in 

 all Echinoderms the skeleton commences in the form 

 of spicules, which gradually unite mere or less with 

 one another, so much as one that lias been secondarily 

 acquired. In some cases (Psolus) the calcareous 

 plates are quite large, firm, and connected, and, on 

 the other hand, the spicules sometimes disappear 

 completely from old and large, even where they are 

 present in younger and smaller, examples of some 

 species of Cucumaria. In Synapta the spicules take 

 on the form of anchors ; in Chirodota, of toothed 

 wheels. They are often turn-form in shape, and the 

 surface of the body is sometimes quite rough to tfie 

 touch, owing to the large numbers which are present 



