64 EMBRYOLOGY OF THE STARFISH. Part I. 



the whole comparison so puerile as not to be worth even a moment's consideration; 

 and the off-hand way, in which he dismisses the whole subject, shows his total want 

 of appreciation of the arguments 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 Cteno- 

 phoraa, 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 larvse in their adult condition resembling young Ctenophorfe. From 

 an examination of drawings given by Midler, 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 be- 

 tween young Trachynemse and Echinoderm larvaa. 



From what has been said, it is evident that the plan of radiation underlies this 

 apparent bilaterality of the Brachiolaria, and of the Pluteus. The throwing of the 

 whole of the stomach and the alimentary 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 Holothurtans 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 Spatangoids. 

 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 radiating spheromeres. The earliest stages of the larvse of 

 Echinoderms, before 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, previous to the 

 formation of tentacles. What constitutes the difference in the structural plan of these 

 animals belonging to different classes, in their primary stages of growth ? They are 

 all built according to one and the same idea, so carried out as to be eminently 

 echinodermoid in one instance, acalephian in another, and polypoidal in a third. In 

 young Echinoderms, as in young Ctenophorse, we find nothing of the remarkable 

 preponderance of certain parts which gives these young their bilateral appearance in 

 more advanced conditions. Their radiate character is extremely prominent at first, 

 but becomes gradually obscured and hidden under the guise of this bilaterality, which 



