184 



THE EYE IN EVOLUTION 



Starfish 



Brittle-star 



be anticipated from the absence of centralization in the nervous 

 system, associated presumably with the absence of a head region, 

 and from the characteristically sluggish and sedentary habits of its 

 members. The phylum is divided into 5 extant classes : 



ASTEROiDEA, or starfishes, motile but sluggish organisms. 



OPHiUKOiDEA or brittle-stars, resembling starfishes but with the arms 

 sharply marked off from the central disc. 



ECHINOIDEA or sea-urchins, living off rocky coasts, with a round pin- 

 cushion-like body covered with plates and provided with long sharp spines. 



HOLOTHUEOiDEA Or sea-cucumbers, worm-like creatures with calcareous 

 plates, occurring in most seas. 



CRiNOiDEA, sea-lilies or feather-stars, stalked forms anchored on rocks or 



Free-swimming 

 feather-star 



Fig. 165. — The Iridophokes ix the .Sea-urchin, Djadema aatjllarum. 



Section through a cluster of iridophores, I ; E, epidermal layer ; M, 

 melanophores, underneath which lies the superficial nerve layer (fixed Bouin ; 

 stained Masson's argentaffine reaction ; counter-stained Mallory's triple 

 stain. (Approx. X 500) (N. Millott). 



in mud usually at great depths, with appendages (cirri) and branching arms 

 growing from a central cujd ; feather-stars become free-swimming in adult life. 



In most Echinoderms the skin is diffusely sensitive to light, 

 particularly in sea-cucumbers (Crozier, 1914-15) ; in brittle-stars and 

 feather-stars there are no special sense organs ; in sea-cucumbers 

 sense organs are represented by statocysts sometimes present at the 

 bases of the tentacles, and tactile processes sometimes present on the 

 dorsal surface of some of the creeping forms ; " eyes " are present only 

 in starfishes. 



The diffuse dermatoptic sense shows interesting variations. Thus in some 

 starfishes the body-surface is said to be sensitive to changes in intensity, the 

 podia and skin gills to steady light ; in some sea-cucumbers {Synapta) the whole 

 sn; •■■ is sensitive to both, while in others {Holothuria surinamensis) the rim of 

 the •■'' ica is particularly sensitive, the posterior end and tentacles less so and 

 the ) ia least. In the sea-urchin, Paracentrotus lividus, the apical poles are 



