ON OUR PRESENT KNOWLEDGE OF THE CRUSTACEA. 49 



The branchial organs, that are so essential to the aeration of the blood in 

 all aquatic animals, are iu the Crustacea appendages attached to the mcmbcr.s 

 belonging either to the pereion or pleon or both. In the lower and terrestrial 

 types, such as the Isopoda, they are connected with the pleon only. In some 

 Stomapods, as Squilla and its allies, we find them attached to the pleopoda 

 as well as the pereiopoda; but in the higher groups they are invariably 

 attached to the pereiopoda only. In the most simple form the branchite 

 exist as mere saccular attachments, whereas in the higher types they become 

 more complicated and voluminous. In the saccular condition they arc held 

 by a small neck pendent from the joint, and are exposed iu the water without 

 protection ; but in the higher Podophthalmous types they are formed of 

 very numerous plates folded close together upon a central stalk, aud would 

 be very liable to injury if not protected by some means. 



The branchiae, therefore, being in their very nature external organs, and 

 attached to the first joints of the several appendages of the pereion, it is 

 self-evident that they could not be covered or protected by their own somite, 

 inasmuch as if it had passed over them the branchial appendages would be- 

 come internal. Their character and constitution would therefore be changed ; 

 they would cease to be external ; in fact they would cease to be branchiae. 



But since the appendages exist as branchia) and are covered and protected, 

 it must follow that if the protection cannot be evolved from the somites to 

 which they are secondarily attached, the covering must be the result of the 

 development of some other somite. 



The somites in their simx^lo conditions have a tendency to overlap one 

 another to an extent that precludes them from permitting any portion of the 

 intermediate structure being exposed. 



Tliat the somites have a tendency to extend in every direction, is very evi- 

 dent from the different proportions and forms they severally undergo iu various 

 genera, and those which compose the carapace exist in all proportions. 



In the Isopoda the cophalon is reduced to the smallest extent in a typical 

 form of Crustacea. In the Amphipoda the cephalon is much larger than 

 in the Isopoda ; but in neither of these is the integumentary covering pro- 

 duced to cover or protect any somite that is not included within its ana- 

 tomical bounds. In the Diastylidas, one of the lowest forms of the Schizopod 

 type (where the branchiffi consist of but one or two pairs of a multicellular 

 form), the tergal projection of the cephalon extends posteriorly over half the 

 pereion ; whereas the lateral walls are anteriorly produced, so as to protect 

 and cover the anterior cephalic appendages. These animals burrow and 

 live in the mud and sand ; and no doubt this development of the carapace 

 forms a good protection to the eyes and antennal organs. Thus we can 

 readily interpret the origin and homologue of the shell-covering in Limnadia, 

 Cj/pris, &c., by supposing a monstrous development of the carapace iu every 

 direction, induced as a protection to a feeble animal that but for this pro- 

 tection must perish in its destructive habitat. 



In Squilla and its allies (the typical form on which Milne-Edwards has 

 based his researches) the carapace does not extend posteriorly beyond its 

 anatomical bounds ; laterally it projects interiorly more so ; but the great 

 size of this plate arises from the large amount of space that exists between 

 the mandibles and the antennae ; and as a carapace it is scarcely more impor- 

 tant than the tergal surface of the cephalon in the Amphipoda. The 

 branchial organs in tlris type of animals are saccular, or more rudimentary iu 

 their condition than the same organs attached to the pleon. The carapace 

 as a covering is not required to protect these branchial organs, wliich are 



1875. E 



