o/ Me Echinodcrms. 13 



ophthalmus, would lead us to hesitate in drawing any very strict 

 conclusions from position and structure to function, iu the ner- 

 vous 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 Echi- 

 nodemis. 



If we were to arrange the Echinoderms according to the natui'e 

 of their larvae, we should have one group formed by the Asteridse, 

 Holothuriadse and Criuoideae [Comatula) ; and another composed 

 of the Ophiuridffi and Echinidae. And if the acute speculation 

 of Prof. E. Forbes, that the pectinated rhombs of the Cystideae 

 answer to the " epaulettes " of the Echinus-larva, be correct, then 

 the Cystideae would, as a sort of permanent form of Echinus-lana, 

 fall into the latter group, in which they would represent the 

 Crinoideae. 



Interesting as are the phagnomena presented by the lanae of 

 the Echinoderms, taken iu themselves, as mere facts, they are 

 far more important in their bearing upon one of the most com- 

 prehensive 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 num- 

 ber of new cases by Steenstrup, and finally reduced to a fixed and 

 definite scientific forai under the name of "Parthenogenesis" 

 by the celebrated Hunterian Professor of Comparative Anatomy, 

 this theory has bid fair to unite all the aberrant generative pro- 

 cesses of the Invertebrata (those of the Echinodenns among the 

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



The theory may be generally expressed thus; 1. The ovum 

 produces an individual A*, whose offspring is another indi\idual 

 B dissimilar to A^ This again may in the same way produce an 

 individual C, and so on. The last of the series only contains ge- 

 nerative organs from which ova are formed, and these reproduce 

 an individual A^ precisely resembling A^ The species therefore 

 is said to be represented by a number of generations of indivi- 

 duals which regularly alternate with one another. 



To this Professor Owen adds — 



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 descend- 

 ants of the embryo cells of the ovum, and which as such, retain 

 a portion of the original "spermatic force," whence they are 

 enabled to attain a certain independent development without a 

 renewal of the spermatic influence. 



* Again, the eyes of the .\cephala are as much supphed from the pal- 

 leal or visceral gangliuu as from the cerebral ganglion. 



