50 RErouT — 1875. 



not more important than the same in the Amphipoda. Gradually, as the 

 branchise assume a more complicated or multicellular condition, the carapace 

 increases in dimensions both laterally and dorsally, until we perceive it 

 reaches the important feature we find in the Brachyurous Crustacea. 



In Squilla the eyes are borne on a distinct somite ; in Palmurus the same 

 is distinctly visible ; in Cancer the ophthalmic somite is likewise distinct and 

 separated from the next succeeding, but it is wrapt over and enclosed by the 

 next or anterior antennal somite. In Squilla also the first pair of antenna) 

 are borne on a somite distinct from the succeeding. In the Macrura and 

 Brachyura this and the succeeding somites are closely blended together ; but 

 in SquiJla the fifth, sixth, and seventh somites are capable of being deter- 

 mined by their sternal pieces only. As we perceive the tergal pieces of the 

 somites of the pereion are wanting in the Brachyura, so we may assume that 

 they are not developed in the posterior somites of the head in Squilla under 

 similar conditions. There therefore is every reason to believe in the theory, 

 that the monstrous development of the mandibidar and posterior antennal 

 somites, incorporated together, unite to form the perfect carapace that is so 

 characteristic of the typical Crustacea. 



But whatever may bo correct in a theoretical or transcendental point of 

 view, for all anatomical and practical requirements the carapace represents 

 the tergal surface of the cephalon, so largely developed as to cover and pro- 

 tect not only the pereion, but, as in Cryptolitliodes, the entire animal. 



In the development of the Crustacea the gradual progress of the carapace 

 may be traced through all its stages. 



In the ovum the members are first represented by small gemmiparous 

 sacs, and precede the foi-mation of the dorsal or ventral arcs in the small 

 Nauplius. The carapace covers and protects all the animal except the pleon ; 

 but this represents only the four anterior somites and their appendages. In 

 the Zoea stage the carapace is perfect and folded downwards laterally, and is 

 capable of covering and protecting all the appendages of the cephalon and 

 the anterior two of the pereion. At this period no branchial organs exist, 

 but saccular appendages in an embryonic condition are budding in their places : 

 in a short time the pereiopoda are seen to form, and the branchial organs 

 assume a definite character ; and with their appearance a change takes place 

 in the form of the carapace. 



In a largo number of Brachyiural ZoUoi a more or less conspicuous spine or 

 tooth-like process may be seen to occupy a position on the lateral walls. This 

 spine, from observation during the progressive growth of the animal, is seen 

 to correspond with the angle in the adult that defines the demarcation 

 between the branchial and hepatic regions. The deflection of the carapace 

 anteriorly bends over the hepatic lobes, the line of the greatest curvature 

 being frequently surmounted by a series of well-defined tooth-like cusps ; and 

 posteriorly bends over the branchial organs, the curvature here being less 

 abrupt and seldom surmounted by any cusp or process. 



Externally the carapace covers and protects both the hepatic and branchial 

 organs ; but internally a calcareous wall of demarcation exists. 



This wall, which Milne-Edwards terms the apodema, is continued into a 

 thin membranous tissue that makes a distinct and weU-defined separation be- 

 tween the branchial appendages and the internal system ; so that the aqueous 

 element, so necessary for the aeration of the blood as it passes through the 

 branchife, may have fuU power to play upon the gills without having any 

 passage that would admit it to the internal viscera and derange the general 

 economy of the animal. 



Not only does the carapace vary in external form, but also in the configu- 



