v CRUSTACEA OUTER ORGANISATION 307 



two respiratory cavities, and are called gill covers or branchiostegites. The form 

 and dimensions of the abdomen are very various. In. the Macrura the abdomen 

 is strongly developed. In the good swimmers (e.g. Carididce) the whole body 

 is laterally compressed, while in those Macrura which generally crawl, or only swim 

 occasionally and not well (Astacidoe, Palinuridcc, Galatheidce, Thalassinidce), it is 

 more or less flattened dorso-ventrally. The abdomen always ends in a strong caudal 

 fin (terminal segment with the limbs of the 6th abdominal segment). In the Paguridtu 

 (hermit crabs), which live in empty mollusc shells, the last thoracic segment is 

 separate and not fused with the cephalo-thorax ; the abdomen, which is covered 

 by the mollusc shell, is soft-skinned, the caudal fin reduced and bent round for- 

 wards. The Brachyura are distinguished by the fact that the abdomen is -reduced 

 to a small plate, which is bent forward on the ventral side of the cephalo-thorax, so 

 that in looking at these Crustaceans from above only the cephalo-thorax can be seen. 

 The abdominal limbs are reduced in number and form. The caudal fin is atrophied. 



B. The Extremities. 



According to the scheme sketched above of the segmentation of 

 the Crustacean body, every segment except the last is provided with a 

 pair of jointed extremities which articulate ventrally and laterally 

 with the body. In order to complete the scheme with reference to 

 the limbs, we must distinguish between the limbs of the most anterior 

 segment and the rest. The first pair are not biramose, but consist of 

 a single row of consecutive joints. All the other pairs are biramose. 

 In such a biramose limb we distinguish 3 parts ; the shaft or stem 

 (protopodite), the inner branch (endopodite), and the outer branch 

 (exopodite). The shaft consists of 2 joints, a proximal joint articu- 

 lating with the body, and a distal joint carrying the 2 branches. 

 The 2 branches themselves are again jointed ; the inner branch is 

 turned towards the median plane of the body, the outer branch away 

 from it. 



The limbs of the Crustacea undergo the most various transforma- 

 tions according to the special functions they perform. They can 

 always, however, be referred back to the typical forms, the first pair 

 to the unbranched (uniramose) form, and all the rest to the biramose 

 form. 



This typical arrangement is found in the Nauplius, which is the 

 youngest Crustacean larva, universally found among the Entomostraca 

 and occurring also in a few Malacostraca. The Nauplius, which 

 hatches from the egg, is unsegmented, and always possesses 3 pairs of 

 appendages, the first uniramose, the second and third biramose. 



The limbs of the Nauplius become in all cases the three anterior 

 pairs of limbs of the adult animal. The most anterior pair becomes 

 the anterior antennae, the second the posterior antennae, and the 

 third the mandibles of the adult. In the course of the metamor- 

 phoses of the Nauplius, which are accompanied by numerous moults, 

 the larval body elongates into the adult animal, and behind the 

 appendages of the Nauplius new appendages bud from the body as a 

 rule in order from before backward. All these newly formed 



