464 ECHINODERMATA ASTEROIDEA chap. 



Asterina lays its eggs in accessible localities, its development 

 has been more thoroughly worked out than that of any other 

 species. Fahnijjes memhranaceus, an animal of extraordinary 

 thinness and flatness, is sometimes dredged up off the coast of 

 Britain in deeper water. Its arms are so short that the general 

 form is pentagonal. The infero-marginal plates are long and 

 rod-like, and form a conspicuous border to the body when viewed 

 from below. 



Fam. 4. Poraniidae. — Spinulosa allied to the Asterinidae but 

 possessing a thick gelatinous body-wall in which the plates and 

 spines are buried, the marginals forming a conspicuous border to 

 the body. This family is represented in British waters only by 

 Porania pulvillus, a cushion-shaped Starfish with very short arms 

 and of a magnificent reddish-purple colour. It is occasionally, 

 but rarely, exposed at low tide. 



Fam. 5. Ganeriidae. — Spinulosa allied to the Asterinidae but 

 distinguished by the large marginals and by the fact that the 

 skeleton of the oral surface consists of plates each bearing a few 

 large spines. Ganeria, Ifarginaster. 



Fam. 6. Mithrodidae. — Spinulosa with a reticulate aboral 

 skeleton. The spines are large and blunt, covered with minute 

 spinules. Ifithrodia, sole genus. 



These last two families are not represented in British waters. 



Order II. Velata. 



This is a very extraordinary group of Starfish, about the 

 habits of which nothing is known, since they all live at very 

 considerable depths. Their nearest allies amongst the Spinulosa 

 must be looked for amongst the Solasteridae. If the sheaves of 

 spines with which the latter family are provided were to 

 become adherent at their bases, and connected with webs of skin 

 so as to form umbrella-like structures, and if then these umbrellas 

 were to become united at their edges, we should have a supra- 

 dorsal membrane formed such as is characteristic of the order. 



Fam. 1. Pythonasteridae. — Velata in which each sheaf of 

 spines is enveloped in a globular expansion of the skin and is not 

 united with the neighbouring sheaves. Pythonaster, sole genus. 



Fam. 2. Myxasteridae. — Velata with numerous arms in 

 which the sheaves of spines are long and form with their con- 



