192 PHYLUM ECHINODERMATA. 



cellariae. Crossaster M. and T. ; Solaster Forbes ; Rhipidaster Slad. ; 

 Ctenaster Perr. ; Lophaster Verrill ; Koreihraster W. Thorns. ; Peribolaster 

 Slad. British species : Solaster papposus, E. N. Atl. to 640 fms., S. endeca, 

 ditto to 150 fms. 



Fam. 5. Pterasteridae. With a reticulated dorsal skeleton bearing 

 paxilliform groups of spines. These spines are united together by a 

 membrane (supradorsal membrane) which forms a continuous canopy over 

 the dorsal surface. The chamber so enclosed is said to be a brood-chamber, 

 and opens to the exterior centrally by a valvular aperture, and by a 

 number of small contractile pores in the supradorsal membrane, and at the 

 side of the arms by apertures regularly recurring over each adambulacral 

 plate and called the segmental apertures. The canopy is present in all 

 specimens hitherto examined ; it is uncertain whether these specimens 

 are females or hermaphrodites or whether the canopy is present in both 

 sexes. The supradorsal membrane may in some forms be wholly or 

 partially aborted. Actinolateral spines when present united by mem- 

 brane so as to form a web on the actinal surface. Pedicellariae absent. 



Pteraster M. and T. ; Eetaster Perr. ; Marsipaster Slad., from the deep 

 sea ; Calyptraster Slad. ; Hymenaster W. Thorns., almost entirely an 

 abyssal form ; Benthaster Slad., from the deep sea ; Myxaster Perr. ; 

 Cryptaster Perr. The above are disco-pentagonal in form and have a 

 supradorsal membrane with segmental apertures. Pythonaster Slad., 

 stellate forms without supradorsal membrane, actino-lateral spines and 

 segmental apertures, from the deep sea. 



Fain. 6. Echinasteridae. Dorsal skeleton formed of plates disposed 

 in longitudinal and transverse series, or in an irregular network bearing 

 spines. Spines moderate, pointed, naked or covered by a thin membrane 

 containing calcareous granulations. Arms long. Pedicellariae present 

 only in Acanthaster and Valvaster. 



Acanthaster Gervais, with numerous arms (more than 10) and several 

 (5-16) madreporic plates ; Mithrodia Gray ; Cribrella Ag. (Henricia 

 Gray) ; Perknaster Slad. ; Echinaster M. and T. (Fig. 118) ; Plectaster Slad. ; 

 Valvaster Perr., with wide-meshed arrangement of the calcareous plates 

 and large groups of papulae. British species : Cribrella sanguinolenta, 

 E. N. Atl., to L350 fms. 



Fam. 7. Heliasteridae. Arms very numerous (more than 25) and 

 short, disc large. Abactinal skeleton reticulate. Tube-feet in four rows 

 Double interbrachial septa. Heliaster Gray. 



Fam. 8. Pedicellasteridae. Disc small, not sharply marked off from 

 the arms. Abactinal skeleton of the arms reticulated. Tube -feet in 

 two rows. With numerous, large forcipiform -pedicellariae. Genital 

 organs open on the disc. Pedicellaster Sars ; Coronaster Perr., with 

 numerous arms ; Lytaster Perr. ; Gastraster Perr. 



Fam. 9. Asteriidae. With reticulate abactinal skeleton, bearing 

 isolated or grouped spines. Tube-feet in four rows. Pedicellariae for- 

 ficiform or forcipiform. Members of this family which have more than 

 five arms are called heteractinides : in such species the number may be 

 constantly six, or the number may be subject to individual variation. 



Pycnopodia Stimpson ; Coscinasterias Verrill ; Polyasterias Perr. ; 

 Stolasterias Slad. ; Leptasterias Verrill ; Asterias L. ; Diplasterias, Perr. ; 

 Smilasterias Slad. ; Sporasterias Perr ; Anasterias Perr. ; Hydrasterias 

 Slad. ; Cosmasterias Slad. ; Podasterias Perr. ; Uniophora Gray ; Cal- 

 vasterias Perr. British species : Asterias glacialis, E. N. Atl., to 66 fms., 



