MOLLUSCS 



247 



they take their English name. In most species the 

 small body scarcely affords room for the attachment 

 of eight (very rarely ten) long slender legs, but in a 

 few cases the legs are short and stout ; as, for instance, 

 in the common Pycnogonum littorale, which, in spite 

 of its name, is often found in moderately deep water. 

 Most of the Pycnogons are of small or moderate size, 

 but in some deep-water species the legs may span a 

 length of 2 feet. 



FIG. 174. ARCHASTER, A STAR-FISH ("BLAKE"). 



The commoner forms of the Mollusca, at any rate of 

 the shelled species, are familiar to everyone, but there 

 are some shell-less species, and others with peculiar 

 shells, which call for special note. The usual type of 

 Gastropod, as exemplified by the whelk or periwinkle, 

 bears an asymmetrically coiled shell, but there is reason 

 to believe that the earliest forms of molluscs were 

 symmetrical. This symmetry is still found in the 

 Chitons, in which the shell is made up of eight plates, 



