300 COMPARATIVE ANATOMY CHAP. 



(fresh water), Cancer, Xantho, Pilumnus, EripJiia, Portunus, Carcinus, in the sea. 

 Catometopa, square crabs : Pinnoteres, Ocypoda, Grapsus, in the sea, Gecarcinus, 

 land-crabs. 



I. Outer Organisation. 1 



Nowhere in the animal kingdom does the study of outer organ- 

 isation afford so much of interest to the comparative anatomist as 

 among the Arthropoda. The more or less hard chitinous envelope which 

 outwardly covers the body and all its limbs not only serves as a 

 protection to the inner organs, but also represents the skeleton to 

 which the musculature is attached inside. Herein lies the chief cause 

 of the specially close relations between inner and outer organisation 

 in the Artkropuda. 



In describing the outer organisation of the Crustacea we must take 

 into consideration in turn (1) the body, (2) the extremities, and (3) the 

 gills. 



A. The Body. 



We denote by this term the whole animal minus its appendages. 

 It consists of a number of consecutive joints (segments, metameres, 

 somites). 



The consecutive segments are movably articulated together. The 

 chitinous integument investing the whole animal remains thin and soft 

 between adjoining segments and forms intersegmental membranes. 



The study of comparative anatomy leads us to suppose that 

 originally each segment except the last possessed a pair of limbs, so 

 that the number of the limbs answered to the number of somites. 

 We may diagrammatically represent the Crustacean body as consisting 

 of a great number of segments, as is the case in the Annulata. The 

 most anterior or head segment is distinguished by the possession of the 

 eyes, the mouth, the brain, and a pair of extremities, which as feelers 

 differ from all the other extremities ; these latter resemble one another 

 more or less closely ; their special modifications will be described below. 

 The anus lies in the hindermost segment, which has no limbs. 



This diagrammatic representation of the segmentation of the 

 Crustacean body is not exactly realised in any known Crustacean. 

 In fact we everywhere find important deviations even in those which 

 are considered to stand nearest the racial form. 



We find first of all that in all Crustaceans the anterior region of 

 the body is outwardly unsegmented, and, in opposition to our typical 

 Crustacean, carries not one but five pairs of limbs. We are inclined 

 to assume that this region has arisen by the fusing of a head segment 

 with the four following segments, this assumption being supported by 

 similar phenomena observed in various groups of Crustacea. 



Thus the most anterior unsegmented region of the body, which 



1 In order to emphasise the great morphological significance of the Leptostracan 

 body, Nebalia, is treated of in this division apart from the other Malacostraca. 



