32 ECHINODERMA GENERAL DESCRIPTION 



In Asteroids (Fig. XXV. 1) an eye-spot (e) lies at the base of each 

 terminal tentacle (t) on its aboral side. This spot is a red cushion 

 in which are many conical cups, each representing an eye (Fig. 

 XXV. 2). The wall of each cup is formed of pigment cells and 

 interspersed unpigmented retinal cells (Fig. XXV. 3). The 

 Echinoid Diadema setosum has a black integument with numerous 

 spots which, owing to interference of light, appear blue. Each 

 blue spot, as proved by P. and F. Sarasin (1887), is a com- 

 pound eye (Fig. XXV. 4). The structure of a single element is 

 shown in Fig. XXVI. Supposed auditory organs ("Baur's 

 vesicles" or otocysts) have been described only in some Holo- 

 thurians, e.g. Synapta (see Fig. XVII. oc; also p. 234 and Fig. V. 5 

 on p. 233). The sphaeridia of Echinoids (see p. 288) are sup- 

 posed to be organs of orientation, or of taste and smell (Loven), 

 or for appreciating chemical changes in the water (Ayers, 1885). 

 They occur only on the oral side of the theca, and when the 

 animal is in the natural position they hang down like the clapper 

 of a bell ; but when the animal is tilted, each sphaeridium presses 

 against the nerve cushion surrounding its stalk. 



Distinctive Characters of Phylum and Classes. The foregoing 

 account has introduced the fundamental features of Echinoderm 

 morphology, laying stress on characters common to the whole 

 Phylum rather than on those that distinguish the various classes. 

 It has, however, tended to show the inner meaning of those out- 

 ward distinctions between the chief types with which the chapter 

 opened, and the student may perhaps have realised that " the 

 homologies within the Echinoderm stock " are, as Semon has in- 

 sisted, often more apparent than real. In drawing up a definition 

 of the Phylum that shall include the most primitive forms of fixed 

 Echinoderms known, one cannot utilise most of the characters 

 usually thus employed in systematic treatises, since they are 

 secondary, homoplastic acquisitions, often with no true homology. 

 It is, for instance, not sure that all Echinoderms have a radiate 

 symmetry, even an obscured one. It is true that all recent 

 Echinoderms have a lacunar, haemal system ; but that system in 

 Stelleroidea is not homologous with the one in Crinoidea. It is 

 highly probable that all animals to which the name " Echinoderm " 

 could have been applicable since the beginning have had a portion 

 or portions of the anterior coelom specialised as a hydrocoel ; but 

 this is different from the questionable assertion that all Echino- 

 derms have an ambulacral system. 



On the other hand, in any attempt to limit the several classes, 

 respect should be paid to deep-seated structures illustrative of past 

 history and genetic affinity rather than to the obvious but super- 

 ficial differentiations that characterise the representatives now 

 living. We have to make our classificatory partitions run back 



