82 ESSAY ON CLASSIFICATION 



without alternate generations, and as some are viviparous their 

 changes can easily be traced. The Cestods and Cystici, which were 

 long considered as separate orders of Helminths, are now known to 

 stand in direct genetic connection with one another, the Cystici 

 being only earlier stages of development of the Cestods. The Trema- 

 tods exhibit the most complicated phenomena of alternate genera- 

 tions; but as no single species has thus far been traced through all 

 the successive stages of its transformations, doubts are still enter- 

 tained respecting the genetic connection of many of the forms which 

 appear to belong to the same organic cycle. It is also still question- 

 able, whether Gregarinae and Psorospermia are embryonic forms or 

 not, though the most recent investigations render it probable that 

 they are. The development of the Annulata, as they are now cir- 

 cumscribed, exhibits great variety; some resemble more the Nema- 

 tods, in their metamorphoses, while others, the Leeches for instance, 

 approximate more the type of the Trematods. The Sipunculoids 

 appear to be more closely related to the Annulata than to the Holo- 

 thurioids. 



The class of Crustacea, on the contrary, may be considered as one 

 of the best known, as far as its zoological characters and embryonic 

 growth are concerned, the only point still questioned being the re- 

 lationship of the Rotifera. In their mode of development the Ler- 

 naeans, the Entomostraca proper, and the Cirripeds agree as closely 

 with one another as they differ from the higher Crustacea. This 

 conformity is the more interesting, as the low position the Entomos- 

 traca hold in the class of Crustacea agrees strikingly with their early 

 appearance in geological times, while the form of the adult Cirripeds 

 and that of the Lernaeans would hardly lead one to suspect their 

 near relationship, which has, indeed, been quite overlooked until 

 Embryology showed that their true position is among Crustacea. In 

 the development of the higher Crustacea their superior rank is 

 plainly exhibited, and few types show more directly a resemblance, 

 in their early stages of development, to the lower members of their 

 class than the Brachyura. 



In the class of Insects I include Myriapods, Arachnoids, and the 

 true Insects, as, according to the views expressed hereafter, these 

 natural groups constitute only different degrees of complication of 

 the same combination of organic systems, and must therefore be 



