﻿Dec, 1857.] 



ELLIOTT SOCIETY. 



267 



may, perhaps, be altogether without representatives in the lower 

 orders. Two circumstances connected with these parts, however, 

 deserve special notice. One is that the ambulacral tubes, or their 

 representatives, and the ambulacra find a connection with the 

 area of the sense-capsule before they stretch downward tow 7 ard the 

 mouth. This shows that the area in question at the superior pole 

 of the body is the point towards which even the serial homologues 

 of the radiate tubes tend, a fact which strengthens the notion that 

 the periphery of this area is the homolog^ie of the disk margin in 

 the lower medusae. The second is that this explanation of the 

 homologies leaves no longer any discrepancy between the position 

 of the sexual organs among Discophores and Hydroids, and its 

 position among Ctenophora. For in both they are situate beneath 

 the external tissues of the same surface, which is within the bell or 

 disk-cavity, among the Hydroids and Discophores, and therefore 

 concave, but which is convex and external among Ctenophora. 



At the same time if we carefully scrutinize the morphological 

 tendencies of the Discophora proper, it will be seen that the cir- 

 cular tube exhibits a constant tendency to be lost in a labyrinth of 

 interlacing tubes around the digestive cavity. And that, in this 

 order, the marginal capsules and tentacula are no longer sessile 

 upon the circular tube, but are provided with special radiate 

 canals, which stretch between the circular canal, or its homologous 

 plexus and the margin of the disk, where each of these canals 

 divides into two more or less short blind branches which embrace 

 the base either of a sense-capsule or a tentaculum, or of a fasci- 

 culus of tentacula. Again — we observe that the tentacula con- 

 tinually tend to shift their position to the under side of the disk. 

 It is, therefore, possible that the large central sinuses in the 

 young Ctenophores represent the whole circulatory system of 

 Hydroidea, including the circular marginal tube, and that ail 

 their radiate tubes (not their superficial tubes,) are homologous 

 with that portion only of the radiate tubes in Discophora, which 

 stretches externally to the circular tube, from the latter to the 

 margin. The peculiar forked termination of these tubes is the 

 same in both Discophora and Ctenophora, so far as those of the 

 sense-capsules and tentacula are concerned. And it should be 

 remembered that those of the ambulacra, which in the young are 

 paired, do at this stage present the same bifurcate tendency as 

 those of the tentacula. Their branches, however, afterwards 

 separate so far as to appear as independent tubes. I have not 



