380 HOMOLOGIES OF THE RADIATA. Part V. 



abactinal area in Starfishes, at once explains tlie position of their eyes at the 

 end of the so-called arras, which correspond to the summit of the ambulacra in 

 the Echini. If, on the other hand, we start from the simple ambulacra of the 

 Synaptoids, and compare them Avith the genuine HolothuriiB, the presence or absence 

 of ambulacral suckers appears only as a further complication of one and the same 

 apparatus ; and, however diversified this system may be, it remains homologous to 

 the simplest radiating chymiferous tubes of the Acalephs, and is, therefore, also 

 homologous to the radiating chambers of the Polyps. The presence of a simple 

 tube extending over the eye, in our common Starfishes, in the prolongation of 

 the main ambulacral tubes, shows further that the position of the eyes in Echi- 

 noderms is identical with that of the Acalephs, in which the eyes are also in 

 the prolongation of the radiating chymiferous tubes, at the base of the primary 

 tentacles. The complication of the ambulacral system of the Echinoderms is very 

 remarkable in some of their types, assuming at times the form and function of 

 gills on its actinal side, and forming ornamental rosettes, of the most diversified 

 patterns, toward its abactinal side. But everywhere the ambulacra preserve their 

 primary relations to the whole plan of structure. Even the most complicated 

 feelers of Cuvieria and Psolus are only actinal modifications of the ambulacra, 

 performing the functions of tentacles. As to the lantern of the Echini, we need 

 only compare it with the chewing apparatus of Solaster endeca, or Echinaster 

 Solaris, or Paulia horrida, to remain satisfied that it consists of a combination of the 

 interambulacral plates nearest to the mouth, movably articulated upon the next 

 immovable plates of the corresponding interambulacral zones. 



A glance at the mode of development of the Radiates may assist in making 

 these comparisons more precise. Every naturalist now knows how very similar 

 young Polyps and young Hydroids are, and, if in connection with this we take 

 into consideration the fact that the young Aurelia is only a transverse section of 

 the body of a Scyphostoma, the internal identity of these animals must be granted. 

 We have here, therefore, the most direct evidence that young Discophorre are 

 Polyp-like. If we further consider the Acalephian character of the Pluteus-like 

 larvae of Echinoderms, we connect also this class with the other two classes upon 

 embryological evidence. But that evidence amounts to a demonstration of their 

 structural identity, when we compare the twin individuals of a Diphyes-chain with 

 the Pluteus-like larv* of an Echinoderm, in which the Echinoderm has begun its 

 development. In the twin Diphyes, one individual has the structure of a sterile 

 Hydroid, while the other is a genuine sexual Medusa, just as a Pluteus, with its 

 young Echinoderm emerging, is a twin, one individual of Avhich is a sterile Acale- 

 phoid, and the other a sexual Echinoderm. The embryological development of the 

 three classes of Radiates shows that they belong to one and the same type. 



