536 



ECHINODERMATA ECHINOIDEA 



This remarkable family is divided by Mortensen into ten 

 genera, based as usnal on the pedicellariae, but taking into 

 account also the shape of the tip of hard material on the spines. 

 Most authors refer the majority of the species to two genera, 

 Fho7^mosonia and Asthenosoma (Fig. 238), recognising also a genus 

 Spcrosoma for one or two aberrant species. Asthenosoma is dis- 

 tinguished by having wide interspaces of membrane between the 



Fig. 238.— Oral view of Asthejwsouia hi/.^trix. x §. (From Wyville Thomson.) 



plates, and by having ten longitudinal folds of the body- wall, two 

 in each radius, in which powerful longitudinal muscles are de- 

 veloped projecting inwards in the radii. The organs of Stewart 

 are very large. In Phormosoma, on the contrary, the interspaces 

 of membrane are very narrow, and the longitudinal folds are 

 thin and membranous and the organs of Stewart are vestigial. 

 Asthenosoma hystrix and Phormosoma placenta have both Ijcen 

 dredged in deep water off the Irish coast. A. tirens, in which 

 there are ectodermic poison - sacs at the bases of the spines, 

 inhabits the Indian Ocean near Ceylon, and was thoroughly 



