48 REPORT — 1875. 



But this portion will be discussed more fully when the structure of the 

 appendages is treated of. 



The earliest stage in the life of a crustaceous animal, in which the dorsal 

 shield known as the carapace is observable, is that of the young as it exists 

 fresh from the ovum of a cirriped (Pl.jl. fig. 1) . This, which has been named the 

 Nauplius form of the Crustacea by Pritz Miiller, exists as a small animal with 

 three pairs of appendages only. The eyes are not developed, the ocular spot 

 not being homologous with the permanent organs ; but since we see that mate- 

 rial does enter into the stomach, we can have no great effort in accepting the 

 proposition that this incipient animal has a mouth ; and such being the case, 

 we must assume that the anterior four somites are present in the construc- 

 tion of the head of the Nauplius stage of Crustacea. The oral apparatus is 

 still in an embryonic condition. 



The next stage of living types in which we can observe the carapace to exist 

 in the progressive condition is in that known as the Zoea form of Crustacea 

 (PI. I. fig. 2). This is the early life of the young of the higher Podophthalmous 

 Crustacea. That of the Brachyura is most known and most instructive. Some 

 of the appendages are begiuniug to assume a permanent form. The eyes are 

 developed, the antennae (though in an immature coudition) are in existence, 

 and so are all the appendages of the head except the last. The first two 

 pairs of appendages connected with the pereion are present in an immature 

 condition, and the posterior pairs are represented by small bud-like appen- 

 dages. Dissection readily demonstrates that the carapace in this stage only 

 covers, but has no associated connexion with, the appendages of the pereion ; 

 and a closer study shows that the heart is connected with and partly exists 

 in the great dorsal spine. The relative position of this process, therefore, 

 enables us to determine that the future growth of the carapace takes place 

 and is connected with the anterior portion of this structure, and not with the 

 posterior. In the young of Palinurus, as well as in the larger forms known 

 as Phyllosoma, which appears to be the young of Palinurus older in age and 

 larger in size, the carapace is developed largely in advance of the oral ap- 

 paratus ; it is produced posteriorly so far as to project over the anterior two 

 somites of the pereion, but is not attached to any portion beyond the posterior 

 oral appendages. An examination of the Zoea of the various types of Podoph- 

 thalmous Crustacea supports this observation; and we can trace the same 

 facts from the Zoea, through the Megalopa, to the adult Brachyurous Crustacea 

 (PL I. fig. 3). It is therefore desirable that we should see how far the study 

 of an adult crustacean will assist lis in demonstrating the true relation of the 

 carapace to the general structure of the animal. 



In Squilla and allied forms of the same type the two anterior somites (the 

 first of which supports the eyes, the second the anterior pair of antennee) exist 

 as distinct and perfect, though small somites ; whereas the two succeeding are 

 closely associated together, and appear as a large dorsal plate supporting the 

 posterior pair of antennae and mandibles. The posterior three somites belong- 

 ing to the cephalon and the first two belonging to the pereion are represented 

 by the sternal plates only. In the young forms the anterior two somites be- 

 longing to the pereion are in a membranous condition dorsally complete. 



According to the theory of Professor Huxley, the carapace represents the 

 dorsal arc of all the somites that it protects and have not a distinct roof of 

 their own. 



It is therefore desirable that we should learn what may be the distinct 

 useful value of the carapace, and why each somite would not serve the same 

 purpose by being perfect in its own arc. 



