TO THE EXISTING ARTICULATA. 33 



from the mere numerical proportion, and to establish the followin«)- g'cneral characters of 

 them : namely, the Crustacea with the numerical proportion 2x5, have always two pair of 

 antennae, compound eyes with facetted cornea, no simple eyes (with a few exceptions, 

 however, as in J/ysv'y), articulated walking feet at the thorax, and always fin feet at the 

 abdomen when this part of the body is present ; the number of articulations never less 

 than three, or exceeding seven. They form the group of the Malacostraca of former modes 

 of division. 



The Crustacea with the fundamental number of three, have fewer general charac- 

 teristics, owing to the very circumstance that they represent a lower division, but I have 

 always found in them compound eyes, with a simple, smooth cornea ;* sometimes also simple 

 eyes, or (occasionally) only the latter, and then in a simple number, especially when young ; 

 they generally have fin feet, and usually in that case no feet at the abdomen, which is fre- 

 quently very short, but in some cases very large ; there is further a remarkable uncertainty 

 in the formation of the antennae and organs of the mouth, the type of which, therefore, is 

 fluctuating. All run through different stages of metamorphosis, and exhibit much greater 

 differences in the respective periods than the members of any other division. I call them 

 Ostracodennata. 



The metamorphosis with its various differences seems to be the circumstance which 

 deserves particular attention in the subdivisions, inasmuch as it exhibits itself partially as 

 retrogressive.t partially as progressive. The retrogressive metamorphosis is not peculiar in 

 an equal degree to all members, since it is occasioned by external circumstances ; and with 

 regard to retrogression, as there is in reality no such process in nature in a strict sense, it 

 cannot be a general character of the whole group, but only an indication of some members 

 of certain sub-groups. If therefore I avail myself of it as a mode of division, I do it only in 

 the same manner as oviparous propagation is mentioned, as a partial characteristic of some 

 of the cold-blooded vertebrate animals, although it does not exhibit itself in all in the same 

 wav. The Ostracodermata, then, are divided into two groups, and each group in three tribes. 

 The absence of a distinct head with true antennae and eyes is as characteristic for the first 

 group, among the members of which a retrogressive metamorphosis is peculiar, just as 

 the presence of very large, frequently monstrously developed eyes is exhibited with a 

 progressive metamorphosis ; and in the same way also very powerfully developed antennae, 

 especially if the eyes become smaller, is characteristic of the second group. The further 

 differences consist in various peculiarities, the explanation of which in detail would occupy 

 too much time ; but I have put them together in a tabular sketch, and by indicating all 

 the higher groups of Crustacea in this table, according to their most important typical 

 characteristics, I have enabled my readers to determine by their own observation how far 

 the Trilobites are related to each, and with which group most intimately. (See the Table, 

 p. 34.) 



* In several species, for instance, in Limulus, it appears facetted «lien in a dried state, but again 

 becomes smooth by being softened with water. 



t The phenomenon of the retrogressive metamorphosis, on \\\\\ch Rathkc has recently written 

 more specially, I Lad availed myself of as a mode of division when giving lectures at Berlin, and have 

 published this view two yeai-s ago, in Ersch and Gruber's Encyclop., vol. xxv, section i, p. 119. 



