Chap. II. HOMOLOGICAL RELATIONS. 65 



to determine the number and the relations of the spheromeres, for in Tiarojjsis^ 

 the eyes are not in the medial prolongation of the radiating chymiferous tubes, 

 though they occupy that position in Coryne (Sarsia) mirabilis and many other 

 Acalephs. Again, in Polyclonia (PI. XIII. Figs. 2, 3, and 4), there are no eyes 

 in the prolongation of the rays in which the sexual pouches are situated, though 

 there is an eye in the medial prolongation of each ray occupied by a sexual pouch 

 in Aurelia and Cyanea (PL IV. and VII). On the other hand, the corners of the 

 mouth always coincide with one radiating chymiferous channel ; and in most Hydroids 

 there are no other chymiferous tubes besides those which thus correspond to the 

 main avenues of the mouth, while the sexual organs follow these channels in 

 bands, on each of their sides, and in all Echinoderms we find the sexual org-ans 

 occupying an interambulacral position. The question, therefore, turns upon this 

 point: Are the spheromeres of Radiates necessarily identical, or may heterogeneous 

 spheromeres alternate with one another? or, in other words: Does the body of an 

 Aurelia consist of eight spheromeres, four of which are connected with the oral 

 appendages and four with the sexual pouches, and that of an Echinus of ten, five 

 of which are ambulacral and five interambulacral? or, are the interambulacral zones 

 only a special expansion of the sides of the ambulacra, and not by themselves 

 distinct zones in the body of Radiates? If we take a comprehensive view of the 

 whole type of Radiates, there seems to me no difficulty in the solution of these 

 questions. In Crinoids and Starfishes the prominent rays of the body are essentially 

 ambulacral in their structure and homologies, and if in Echinoids the interambulacra 

 assume an apparent independence, it is solely owing to the widening of the little 

 plates extending along the ambulacral plates of the Starfishes, and the consequent 

 swelling of the whole body into a more spheroidal form; but even here the 

 so-called interambulacra are only the flanks of the ambulacral zones, and owe their 

 prominence more to the circumscription and separate development of the plates of 

 which they consist than to any intrinsic importance, since nothing of the kind exists 

 in the Holothurians. And if we extend the comparison to Polyps, we see this 

 conclusion fully sustained by the fact, that the radiating partitions, which separate 

 the radiating chambers, bear the same relations to these chambers and their peri- 

 pheric tentacles, as the interambulacra of the Echinoderms bear to the ambulacra; 

 or, in other words, we become satisfied that the radiating chambers are homolooous 

 to the ambulacral system, and the radiating partitions homologous to the inter- 

 ambulacra. Now in Polyps, as well as in Echinoderms, the sexual organs alternate 

 with the ambulacra, that is to say, in Polyps they are attached in a double row 



' See my Contributions to the Nat. Hist, of Figs. 1, 3, 4, and .5, in Mem. Amer. Aead. vol. 4, 

 the Acalephaj of North America, Part. I. PI. VI. and the chapter on Tiaropsis in this vohime. 

 VOL. IV. 9 



