86 ESSAY ON CLASSIFICATION 



The classes of Crustacea and of Insects^^*^^ are particularly instruc- 

 tive in this respect. While the embryo of the highest Crustacea, the 

 Brachyura, resembles by its form and structure the lowest types of 

 this class, as the Entomostraca and Isopoda, it next assumes the shape 

 of those of a higher order, the Macroura, before it appears with all 

 the characteristics of the Brachyura. 



Embryology furnishes also the best measure of the true affinities 

 existing between animals. I do not mean to say that the affinities of 

 animals can only be ascertained by embryonic investigations; the 

 history of Zoology shows, on the contrary, that even before the study 

 of the formation and growth of animals had become a distinct branch 

 of physiology, the general relationship of most animals had already 

 been determined with a remarkable degree of accuracy by anatomical 

 investigations. It is nevertheless true that in some remarkable in- 

 stances the knowledge of the embryonic changes of certain animals 

 gave the first clue to their true affinities, while in other cases it has 

 furnished a very welcome confirmation of relationships, which before 

 could appear probable but were still very problematical. Even Cu- 

 vier considered, for instance, the Barnacles as a distinct class, which 

 he placed among Mollusks, under the name of Cirripeds. It was not 

 until Thompson had shown, what was soon confirmed by Burmeister 

 and Martin St. Ange,^^^ that the young Barnacle has a structure and 

 form identical with that of some of the most common Entomostraca, 

 that their true position in the system of animals could be determined; 

 when they had to be removed to the class of Crustacea, among Ar- 

 ticulata. The same was the case with the Lernasans, which Cuvier 

 arranged with the Intestinal Worms, and which Nordmann has 

 shown upon embryological evidence to belong also to the class of 

 Crustacea. ^^^ Lamarck associated the Crinoids with Polypi, and 

 though they were removed to the class of Echinoderms by Cuvier be- 

 fore the metamorphoses of the Comatula were known, the discovery 

 of their pedunculated young furnished a direct proof that this was 

 their true position. 



"'"' It is expected that Embryology will furnish the means of ascertaining the relative 

 standing of every family. 



"^John W. Thompson, Zoological Researches (6 pts., London, 1828-1834); Hermann 

 Burmeister, Beitrdge zur Naturgeschichte der Rankenfiisser. Cirrepedia (Berlin, 1834); 

 G. J. Martin St. Ange, Memoire sur I'organisation des Cirripedes (Paris, 1835). 



"^Alexander von Nordmann, Micrographische Beitrdge ... (2 vols., Berlin, 1832). 



