PLAN OF DEVELOPMEXT OP^ ECHINODERMS. 81 



seems to consider the whole comparison so puerile as not to be worth 

 even a moment's consideration ; and tlio ofi-hand way in which he dis- 

 misses the whole subject shows his total want of appreciation of the argu- 

 ments by which this view is supported. If the writer of the said article 

 had ever seen the young of Brachiolaria, of Pluteus, or, still better, the 

 young of Tornaria, swimming about amongst crowds of young Ctenophorae, 

 such as Idyia, Pleurobrachia, Mertensia, or Bolina, he would not have 

 passed such a sweeping judgment on this comparison. The motions of 

 a Tornaria are so similar to those of young Ctenophorae, that I venture 

 to say that many a skilful naturalist would be deceived as to their true 

 nature, on first seeing them moving about together in the water. The 

 Tornaria has no appendages developed into long arms as in the adult 

 Brachiolaria or Pluteus. The appendages remain always abortive, the 

 larvae in their adult condition resembling young Ctenophorae. From an 

 examination of drawings given by MUller, Professor Agassiz was induced 

 to make the same comparison already hinted at by Baer, and we have 

 seen that it is sustained in every particular. Gegenbaur has also noticed 

 the resemblance between young Trachynemae and Echinoderm larvae. 



From what has been said, it is evident that the plan of radiation un- 

 derlies this apparent bilaterality of the Brachiolaria and of the Pluteus. 

 The throwing of the whole of the stomach and the alimentarv canal on 

 one side, the complicated system of arms arranged with perfect symmetry 

 on each side of the axis, passing through the mouth and the anus, does 

 not change, though it partially conceals, the radiate plan. We have Holo- 

 thurians which always creep upon three of their ambulacra, where a dorsal 

 and a ventral side, an anterior and a posterior region, are subordinate to 

 the plan of radiation ; and the same takes place to a less extent in Spa- 

 tangoids. Among Polyps even, which are, as it were, the simplest type 

 of radiate animals, an anterior and a posterior region are strikingly shown 

 in the case of Arachnactis. The additional spheromeres are all added at 

 one extremity of the mouth-slit, and yet the Actinia is made up of radi- 

 ating spheromeres. The earliest stages of the larvae of Echinoderms, be- 

 fore the appearance of the water-tubes, reminds us forcibly of the young 

 Actinia soon after it has escaped from the egg, or of the first stages of 

 growth of a Scyphistoma, after it has attached itself to the ground, pre- 

 vious to the formation of tentacles. Let us noAv consider what constitutes 

 the difference in the structure of these animals in their primary stages 



