CHAPTER VII. 



SEA-URCHINS, STARFISH, AND BRITTLE-STARS. 



General characters of common starfish The characters of Echino- 

 derms and the classes The starfishes ami their colour varieties 

 The brittle-stars and their peculiarities of structure The sand- 

 stars Methods of preserving starfish and brittle-stars The sea- 

 urchins Characters of regular urchins Structure of the shell 

 Internal anatomy The heart-urchin, its habitat and structure 

 Contrast with regular urchins The Holothurians Cucumaria and 

 Synapta Development of Echinoderms. 



IN the present chapter we shall be concerned with a very 

 interesting group of animals which are singularly well 

 defined, and not closely related to any others. Some of the 

 general characters of the Echinoderms have already been 

 noticed, others will appear during the course of a prelimi- 

 nary examination of the common starfish, or five-finger. 

 This is to be found in abundance on the shore, especially 

 in the vicinity of mussel beds. It feeds on bivalves of 

 various kinds, and does great damage to mussel and oyster 

 beds. In the neighbourhood of these it grows to a great 

 size, specimens measuring a foot from tip to tip of opposite 

 rays not being uncommon; but on the shore rocks, away 

 from such an extensive food-supply, the usual size is much 

 less. In collecting specimens for examination, you are 

 certain, sooner or later, to encounter individuals strikingly 

 different from the normal. They may have one large ray 

 and four small, or any combination of small and large rays. 

 These illustrate one of the striking peculiarities of the 

 Echinoderms their capacity for regenerating lost parts. 

 In many cases, notably in the brittle-stars, the animals 

 throw off portions of their bodies when attacked ; in other 

 cases, though the animals do not practise self-mutilation to 



125 



