68 DISCOPHORiE. Part 111. 



importance, witli every step of our progress in the knowledge of their structure, 

 just as similar distinctions among MoUusks have lost their value as tests of the 

 natural affinities of these animals. 



If we next consider the systems of radiating tubes, it must be borne in mind 

 that the Echinoderms have not only ambulacral tubes, as is believed, but also, in 

 some of their repi-esentatives at least, jDeculiar interambulacral tubes, quite as ex- 

 tensive as those of the Acalephs, even though these tubes have generally been 

 either overlooked or considered as belonging to the ambulacral system proper. In 

 my third monograph, which is to contain the Natural History of the North American 

 Echinoderms, I shall give a full account of the structure and connections of this 

 complicated system. It may suffice for the present to show, that there exists a 

 system of radiating tubes in the interambulacral zones of the Echinoderms, corre- 

 si^onding to the system of chymiferous tubes radiating from the sexual pouches of 

 the Acalephs to the periphery of the disk, where it anastomoses with the circular 

 tube of the margin, and through this with the ambulacral system, in the same 

 manner as the interambulacral system of radiating tubes of certain Echinoderms 

 anastomoses with a similar circular tube of the margin of their disk, and through 

 this with the ambulacral system proper. This system of radiating tubes is nowhere 

 more extensive, among Echinoderms, than in the families of the Scutellidse, the 

 Clypeastroida?, and the Laganidce ; but the resemblance with the Discophora3 is 

 particularly striking in the Scutellidaj, where the broad expansion of the margin 

 of the disk leads to an obvious similarity of form to the flat disk of our common 

 Meduste. When tracing these homologies, however, it should not be forgotten that, 

 like Starfishes, the DiscophorsB have a broad abactinal area, in consequence of which 

 the whole ambulacral and interambulacral area is brought down to the lower surface 

 of the body; while in the Echinoids the ambulacral and interambulacral zones extend 

 over the sides of the spherosome, and occupy nearly its entire surface, the abactinal 

 area Iseing limited to a comparatively small space, occupied )jy the ovarian and 

 ocular plates and the apparatus which, in different families of Echinoids, may be 

 connected with that region. To facihtate these comparisons, it is, therefore, indis- 

 pensable to assume that some of the parts seen from the dorsal side of an Echi- 

 noid may be brought to the peripheric margin, and even to the lower side of the 

 animal without modifying their homological relations. 



Of all the systems of organs, the ambulacra, with their diversified appendages, 

 are the most characteristic in Echinoderms, and, therefore, the most likely to form 

 a true basis in the appreciation of these homologies. In all Echinoderms, the 

 most important pai'ts of that system ai'e about the mouth, around which they form, 

 at a greater or less distance from the oral aperture, a ring with radiating branches, 

 extending more or less towards the opposite pole of the body, in different families. 



