466 ECHINODERMATA — ASTEROIDEA chap. 



necting " umbrellas " web-like expansions which do not fuse with 

 one another. Myxaster, sole genus, 



Fam. 3. Pterasteridae. — Velata in which the membranes 

 supported by the sheaves of spines are united so as to form a 

 continuous supra-dorsal tent. The Pterasteridae are represented 

 in British waters by a single species, Pteraster militaris, which 

 is occasionally dredged in deep water off the British coast, and is 

 found also in the Norwegian fjords and off the east coast of 

 Canada. This interesting Starfish has five short, blunt arms, and 

 its general appearance at first sight recalls that of Asterina. 

 Closer inspection reveals the " false back." The anus is sur- 

 rounded by five fan-like valves, supported by spines (Fig. 198), 

 underneath which is a space in which the young complete their 

 development, Pteraster being one of the genera in which the 

 normal larval form is not developed. The tendency towards the 

 union of adjacent spines by webs is deeply rooted in the organisa- 

 tion of the animal. It is seen on the under side where the spines 

 borne by the ventral plates are united so as to form transverse 

 combs. In Hymenaster (Fig. 199) the spines borne by the ventral 

 plates are long and free. 



Order III. Paxillosa. 



This is an exceedingly well-marked order. The armature of 

 the upper surface consists of paxillae. These organs as already 

 mentioned are probably to be traced back to sheaves of spines 

 like those of the Solasteridae. The same end as that striven 

 after in the case of the Velata has been attained, but in a 

 different way. The horizontal spinelets of the paxillae meet one 

 another and form a close-fitting mail which is almost as efficient 

 a protection as the webs and umbrellas of the Velata. Pedi- 

 cellariae are occasionally present, but they are always of the 

 pectinate or piucer variety, never forcipulate. 



Fam. 1. Archasteridae. — Paxillosa in which the anus is 

 still retained and in which the tube-feet have suckers. 



The Archasteridae are a most interesting family. Thus 

 Pararchaster has no true paxillae, but only small isolated groups 

 of spines. The pectinate pedicellariae are composed each of two 

 parallel rows of somewhat smaller spines. The members of this 

 family are to some extent intermediate in structure between the 



