ECHINODERMS 



The animals here included constitute a well-marked group of 

 marine animals, in which the body is typically star-shaped or 

 spheroidal, or less commonly cylindrical. There is a more .or 

 less perfect calcareous skeleton developed in the skin, and part 

 of it may consist of fixed or movable spines, hence the name of 

 the group (Gk. echinos, a hedgehog ; derma, skin : i.e. hedgehog- 

 skinned). A peculiar system of tubes is present, containing sea- 

 water and communicating with the exterior. In three of the 

 constituent classes (star-fish, sea-urchins, and sea-cucumbers) part 

 of this "water- vascular" system consists of sucker-like tube-feet, 

 which are used for executing crawling movements. 



Four common British forms are figured as types of the four 

 classes now dominant, i.e. Star-fish (Asteroidea), Brittle Stars 

 (Ophiuroidea), Sea-urchins (Echinoided), and Sea-cucumbers (Holo- 

 thuroidea). 



1. Purple-tipped Sea-urchin {Echinus miliaris}. Body covered 



by numerous spines attached by ball-and-socket joints to 

 little knobs. (" Urchin" is an old name for the hedge- 

 hog.) 



2. Black Sea-cucumber (Holothtiria nigrd). Leathery skin, in 



which scattered calcareous plates are embedded. 



3. Brittle Star (Ophiura). Disc-like body with five flexible 



limb-like arms. 



4. Common Star-fish (Asterias rubens). With five arms which 



are extensions of the central body. ('The large specimen 

 figured has more than the usual number of arms.) 



