Sect. II. SPECIAL HOMOLOGIES. 377 



SECTION II. 



SPECIAL HOMOLOGIES OF THE CLASSES. 



This may give a general idea of the plan of structure of Radiates in general. 

 The three classes of this type differ only in the mode of execution of this plan ; 

 and if I succeed in showing that the whole structure of Echinoderms is strictly 

 homological to that of the Acalephs and Polyps, I shall have proved that these 

 three classes belong to one and the same branch, and that it is unnatural to 

 separate tlie Echinoderms as a distinct type. The structure of the Polyps, as a 

 clas.s, is characterized hy the great uniformity of their spheromeres, which may be 

 considered as hollow, spherical wedges, on the actinal side of w'hich the cavity is 

 prolonged externally into a tentacle. The wide cavity of their spheromeres repre- 

 sents the ambulacral system of the Echinoderms, and the radiating partitions the 

 interambulacral system. The ambulacra of the Polyps diflFer only in being open 

 along the vertical axis, to form the main cavity of the body ; but the peripheric 

 part of this system is even more complicated in some Polyps than in Synapta. 

 In Actinia, for instance, we have a row of distinct pores, opening into the cham- 

 bers, which extend from the tentacles to the foot, and frequently assume the form 

 of distinct papillas or rudimentary tentacles ; while the genital organs hang from 

 the free margin of the radiating partitions. The distinctive character of the Polyps 

 consists, therefore, in the great width of their open ambulacral system, and the 

 narrow interambulacra, projecting as partitions into the main cavity of the body. 

 The number of these spheromeres, the form and number of their tentacles, the 

 presence or absence of solid deposits in their tissues, the mode of branching of 

 the compound communities, affect in no way these homologies. But there are 

 two points in tlie structure of the Polyps which are of special interest with 

 reference to their homologies : the stomach and the small holes on the actinal 

 side of the radiating partitions, through which adjoining chambers communicate with 

 one another. These holes are homologous to the marginal circular tube of the 

 Acalephs, and are actually to be considered as short tubes through narrow walls, 

 leading into wide radiating chambers; as in Acalephs, they are comparatively 

 long tubes througli thick walls, leading into narrow radiating tubes. The manner 

 in which the radiating tubes of the yEquoridas open into the main cavity proves 

 that we have here homological organs. The so-called stomach of the Polyps in 

 no way corresponds to the digestive cavity of the Acalephs ; it is strictly homo- 

 logous to the so-called arms of the Jelly-fishes, only that instead of projecting 



