FUNDAMENTAL RELATIONS OF ANIMALS 113 



Cephalopoda affords but a few data for such comparisons. It is, never- 

 theless, worthy of remark, that while the young Lamellibranchiata 

 are still in their embryonic stage of growth, they resemble, externally 

 at least, Brachiopods more than their own parents, and the young 

 shells of all Gasteropods known in their embryonic stage of growth, 

 being all holostomate, recall the oldest types of that class. Unfor- 

 timately, nothing is yet known of the embryology of the Chambered 

 Cephalopoda, which are the only ones found in the older geological 

 formations, and the changes which the shield of the Dibranchiata 

 undergoes have not yet been observed, so that no comparisons can 

 be established between them and the Belemnites and other represen- 

 tatives of this order in the middle and more recent geological ages. 



Respecting Worms, our knowledge of the fossils is too fragmentary 

 to lead to any conclusion, even should our information of the em- 

 bryology of these animals be sufficient as a basis for similar com- 

 parisons. The class of Crustacea, on the contrary, is very instructive 

 in this respect; but to trace our comparisons through the whole 

 series it is necessary that we should consider simultaneously the em- 

 bryonic growth of the higher Entomostraca, such as Limulus, and 

 that of the highest order of the class, when it will appear that as the 

 former recall in early life the form and character of the Trilobites, 

 so does the young Crab, passing through the form of the Isopods and 

 that of the Macrouran Decapods before it assumes its typical form 

 as Brachyouran, recall the well-known succession of Crustacea 

 through the geological middle ages and the tertiary periods to the 

 present day. The early appearance of Scorpions in the Carboniferous 

 period is probably also a fact to the point, if, as I have attempted to 

 show, Arachnidians may be considered as exemplifying the chrysalis 

 stage of development of Insects; but for reasons already stated (Sect, 

 xxiv) it is hardly possible to take Insects into consideration in these 

 inquiries. 



In my researches upon fossil Fishes I have pointed out at length 

 the embryonic character of the oldest fishes, but much remains to 

 be done in that direction. The only fact of importance I have learned 

 of late is that the young Lepidosteus, long after it has been hatched, 

 exhibits in the form of its tail, characters thus far only known among 

 the fossil fishes of the Devonian system. It is to be hoped, that the 

 embryology of the Crocodile will throw some light upon the succes- 



