76 REPORT — 1876. 



approximate; consequently they are frequently found fused with the anterior 

 somites of the i^ereion. 



Yet in this very genus, in a young state, we have the most complete evidence 

 of the limits that define the cephalon from the percion, and this again from 

 the pleon. 



In the larva of Pcduuims, as well as in the animal known as Fhijlhsoma, 

 which is now generally accepted as being the young of Pcdimirus after some 

 weeks' growth, the cephalon is seen to coincide with the limits of the cara^ 

 pace and terminates anteriorly to the seven somites of the pereion. It there- 

 fore appears tliat it is desirable to identify these first seven somites as belong- 

 ing to the head or cephalon and that only. 



The pereion, or thorax, is also composed of seven somites or segments; and 

 this number is never departed from, even in the most depauperized condition 

 of the animal. These several somites Prof. Milne-Edwards, in his " Obser- 

 vations sur Ic Squelette tegumentaire des Cnistaces dccapodes, et sur la 

 Morphologic dc ccs animaux," Ann. des Sciences Nat. p. 268, 1854, says : — ■ 

 " In order to determine easily each of these anatomical elements of the integu- 

 mentary skeleton, it is desirable to define them by a name ; and I shall call 

 them jyi'otosomite, deutosomite, mesosomiie, or iritosomite, tetartosomite, pemp- 

 tosomite, Jiectosomite, and hehdosomite, following the order which they occupy 

 from before to behind." 



In the lower types they form, as in the Amphipoda, separate and distinct 

 segments ; but in the higher groups, as we see the dorsal surface of the somites 

 of the cephalon developed and produced posteriorly so as to cover and protect 

 the upper part of the pereion, so we find the somites of this latter division 

 coalesce ventrally more or less perfectly until in the Macrura and Brachyura 

 they reach the highest degree of consolidation and are much more dense and 

 strong than is the structure of the carapace. 



This condition is gradually seen to be approached through different stages 

 from the Edriophthalmia upwards. In the genus SqiuUa (which has many 

 analogies with the sessile- eyed Crustacea, and appears like an enormous 

 stalk-eyed Amphipod) three or four of the posterior somites are exposed 

 beyond the carapace and have the dorsal arc complete and separately perfect. 

 In the Diastylida) we see the same : and ultimately in the genus Paguriis, 

 among the Anomurous Crustacea, there is but a single somite that is not 

 embraced within the limits of the carapace, and that is reduced to a very 

 slender ring. 



AVith the deterioration of tlie dorsal arc of each somite of the pereion the ven- 

 tral arc increases in density and coalesces the more perfectly with its neighbours. 

 This appears much to depend upon the habits and character of the animal. 

 If it be one whose habits arc perambulatory, as in Pcdinwus, the somites 

 are strongly fused together into a strong broad sternum ; whereas in such 

 animals as Palcemon and Homarus the sternum is less strongly developed, 

 and apparently of a more feeble character. 



This depreciation of the sternum gradually goes on as we approximate the 

 short-tailed orders, and arises from the absorption of the first joint or coxa 

 of the leg into the general system of the animal. 



In Palinurus the sternum (PI. II. fig. 1), corresponding to the posterior 

 five somites, is very broad, and the legs are very widely separated from those 

 on the opposite side ; in Homarus, Neplirops, and Astacus {V\. II. fig. 2) they 

 approximate each other so nearly that the sternum consists of a small cal- 

 careous longitudinal cord, to which the apodema are attached and receive 

 their support. 



