Sect. II. SPECIAL HOMOLOGIES. 379 



the degree of complication of the proboscidal and of the genital apparatus. All 

 these complications constitute only characteristic features of the subordinate divisions 

 of the class, and in no way influence the homologies. The polymorphism of the 

 Hydroids and Siphonophorse, rightly considered, sets this question completely at rest. 



The character which at first sia-ht distino-uishes the Echinoderms from the Aca- 

 lephs and Polyps is the individualization of all their systems of organs, connected 

 with a striking histological differentiation. This, in a measure, obliterates the impres- 

 sion of similarity which binds them closely together ; in the same way as, for a time, 

 the presence or absence of a shell among Mollusks prevented naturalists from 

 perceiving their closer affinities. But as soon as we can free ourselves from the 

 belief that histological complication and structural differentiation are positive tests 

 of homological relationship, and as soon as we allow due weight to embryological 

 evidence, the close affinities of the Echinoderms and the other classes of Radiates 

 become self-evident. A comparison of a Synapta with a Beroid is most likely to 

 remove at once the impression of a typical difference between these animals. 

 Here we have, in both cases, a cylindrical body, with radiating tubes extending 

 from pole to pole, connected by a circular tube, but without ambulacral suckers. 

 In both, these ambulacral zones alternate with more or less developed interam- 

 bulacra. In none of the members of these types is the body-wall remarkable 

 for its solidity or rigidity. And if the Beroids do not afford direct means of 

 extending the comparison to the tentacles, we need only recall some other Aca- 

 lephs to show that their marginal tentacles are strictly homological to the feelers 

 which in Holothurians surround the mouth, while some other Echinoderm may 

 show us that, as in Radiates generally, the genital organs alternate with the ambu- 

 lacral system, and occupy an interambulacral position. The only important differ- 

 ences between the Echinoderms and Acalephs consist in the isolation of the digestive 

 apparatus from the main mass of the body, forming its outer wall, and the cor- 

 responding isolation of the ambulacral and genital systems; but these differences 

 ai'e only class characters ; they have no reference to the plan of structure. 



This once settled, the special homologies of the Echinoderms are easily traced. 

 The chief difficulty rests with the ambulacral suckers and so-called gills and lantern 

 of the Sea-urchins, and with the position of the eyes in Starfishes, when compared 

 to Echini. These difficulties are, however, readily removed, when the differentiation 

 of the body-wall is taken into consideration. In Crinoids and Starfishes, the 

 abactinal area is very extensive and made up of solid plates, entirely different 

 from those of the actinal area, which consists of the well-known ambulacral and 

 interambulacral plates, occupying nearly the whole surface of the body in Echini, 

 so that their abactinal area is very small, and limited to the narrow space inter- 

 vening between the ocular and ovarian j)lates. The great extension of the 



