FUNDAMENTAL RELATIONS OF ANIMALS 83 



considered as natural orders of one and the same class. This class, 

 though very extensively studied in a zoological and anatomical point 

 of view, and as far as the habits of its representatives are concerned, 

 still requires, however, much patient work, as the early embryonic 

 development of these animals has been much less studied than their 

 later transformations. The type of the Arachnoids embraces two 

 groups, the Acari and the Arachnoids proper, corresponding respec- 

 tively in this class to the Entomostraca and the higher Crustacea. 

 The embryo of the Acari resembles somewhat that of the Entomo- 

 straca, whilst that of the true Spiders recalls the metamorphosis of 

 the higher Crustacea. On the ground of the similarity of their young, 

 some animals, formerly referred to the class of Worms, are now con- 

 sidered as Arachnoids; but the limits between the aquatic Mites and 

 the Pycnogonums are not yet quite defined. 



In the branch of Vertebrata all classes have been extensively 

 studied, and as far as the principal types are concerned, the leading 

 features of their development are satisfactorily known. Much, how- 

 ever, remains to be done to ascertain the minor modifications char- 

 acteristic of the different families. It may even be that further in- 

 vestigations will greatly modify the general classification of the whole 

 branch. The class of Fishes may require subdivision, since the de- 

 velopment of the Plagiostoms differs greatly from that of the ordinary 

 fishes. As it now stands in our systems, the class of Fishes is cer- 

 tainly the most heterogeneous among Vertebrata. The disagreement 

 of authors as to the limits and respective value of its orders and 

 families may be partly owing to the unnatural circumscription of 

 the class itself.^"^ As to the Reptiles, it is already certain that the Am- 



^"^ The peculiarities of the development of the Plagiostoms consist not so much in 

 the few large eggs they produce, and the more intimate connection which the embryo 

 of some of them assumes with the parent, than in the development itself, which, not 

 withstanding the absence of amnios and an allantois, resembles closely in its early 

 stages that of the Reptiles proper and of the Birds, especially in the formation of the 

 vascular system, the presence of a sinus terminalis, etc. Again, besides the more obvious 

 anatomical differences existing between the Plagiostoms and the bony Fishes, it should 

 be remembered that, as in the higher Vertebrata, the ovary is separated from the ovi- 

 ducts in the Sharks and Skates, and the eggs are taken up by a wide fallopian tube. 

 That the Plagiostoms can hardly be considered simply as an order in the class of 

 Fishes, could already be inferred from the fact that they do not constitute a natural 

 series with the other Fishes. I would, therefore, propose the name of selachians for a 

 distinct class embracing the Sharks, Skates, and Chimaeras. Recent investigations upon 

 the Cyclostoms show them also to differ widely from the Fishes proper, and they too 

 ought to be separated as a distinct class, for which the name of myzontes may be most 

 appropriate. 



