ANATOMY AND DEVELOPMENT OF THE ECHINODERMS US 



in the Annelida is truly stomatogastric. But in the first place it has 

 not been shown so to be ; and in the second place, the existence of 

 well organized eyes supplied by nerves from the ordinary ventral 

 ganglia in each segment of Polyophthabnus, would lead us to hesitate 

 in drawing any very strict conclusions from position and structure to 

 function, in the nervous system of these animals.'^ 



Yet one word upon the bearing of the facts of development now 

 made known, on the affinities of the various groups of Echinoderms. 



If we were to arrange the Echinoderms according to the nature of 

 their larvc-e, we should have one group formed by the Asterida;, Holo- 

 thuriada; and Crinoideae {Coinatuld) ; and another composed of the 

 Ophiuridae and Echinidae. And if the acute speculation of Prof E. 

 Forbes, that the pectinated rhombs of the Cystideas answer to the 

 " epaulettes " of the Echinus-larva, be correct, then the Cystideae would, 

 as a sort of permanent form of Echinus-larva, fall into the latter group, 

 in which they \vould represent the Crinoideae. 



Interesting as are the phaenomena presented by the larva; of the 

 Echinoderms, taken in themselves, as mere facts, they are far more 

 important in their bearing upon one of the most comprehensive and 

 interesting zoological theories of modern times — we refer to the theory 

 •of " the alternation of generations." Founded by Chamisso and 

 Eschscholz, extended to a great number of new cases by Steenstrup, 

 and finally reduced to a fixed and definite scientific form under the 

 name of " Parthenogenesis" by the celebrated Hunterian Professor of 

 Comparative Anatomy, this theory has bid fair to unite all the aberrant 

 generative processes of the Invertebrata (those of the Echinoderms 

 among the rest) under its conditions, and to express them in its terms. 



The theory may be generally expressed thus : — i. The ovum pro- 

 duces an individual A^, \\ hose offspring is another individual B dis- 

 -similar to A^. This again may in the same way produce an individual 

 C, and so on. The last of the series only contains generative organs 

 from which ova are formed, and these reproduce an individual A^ pre- 

 cisely resembling A^- The species, therefore, is said to be represented 

 by a number of generations of individuals which regularly alternate 

 with one another. 



To this Professor Owen add.s — 



2. That the individuals B, C, D, &c. which intervene between the 

 sexual individuals A^ and A^ are always developed from masses of 

 cells which are the immediate and unchanged descendants of the 



1 Again, the eyes of the Acephala are as much supplied from the palleal or visceral gan- 

 glion as from the cerebral ganglion. 



I 2 



