96 Descriptions of Preparations. 



and uniting the anterior extremities of the two laterally placed 

 lines. The area on the omo-stegite intercepted between the curved 

 line and the cervical suture represents the terga of the three 

 thoracic segments ; the area posteriorly to this line represents the 

 terga of the five abdominal segments. The portions of the omo- 

 stegite which lie laterally to these mesial areae are the connate 

 pleura of all the eight segments just mentioned; from their 

 function they are called ' branchio-stegites.' Their free border con- 

 sists of a smooth rim, thickly fringed with hairs, arising along its 

 inner edge ; between which and the coxopodites of the thoracico- 

 abdominal segments, water can find free access to the branchial 

 chamber. The branchial portions of the carapace may be considered 

 as representing the fused pleura of the eight thoracico-abdominal 

 segments. The cervical suture which separates the omo-stegite 

 from the cephalo-stegite begins anteriorly opposite the middle hne 

 of the antennary sternum. From this point it passes at first 

 horizontally backwards ; then it turns almost vertically upwards, 

 bounding a surface of the omo-stegite, which is slightly convex 

 forwards, and beset with from four to five spines ; finally, it bends 

 boldly backwards, describing a curve the mesial portion of which 

 bounds the cardiac area anteriorly. The cephalo-stegite is pro- 

 longed anteriorly into a triangular mesial rostrum, terminating 

 anteriorly in a sharp point, about on a level with the commence- 

 ment of the antennary flagellum. The cephalo-stegite carries a 

 sharp spine on either side, just externally to the basis of the 

 rostrum. Immediately posteriorly to this spine is seen a convex 

 surface, which is conspicuous even in early stages of development, 

 and marks the origin of the powerful adductor mandibulae muscle. 

 Inferiorly the cephalo-stegite is connected, though it is not anchy- 

 losed, with the antennary sternum, as it is in the Brachyura, and 

 indeed in the much more nearly allied species Homarm Vulgaris. 



For a full description of the external and internal anatomy of 

 Astacus Fluviatilis, see Huxley, Medical Times and Gazette, 

 Feb. 7, 1857 seqq. ; and for the tegumentary skeleton and 

 morphology of Decapodous Crustacea generally, see Milne- 

 Edwards, Ann. Sci. Nat., Ser. iii., tom. xvi., p. 221, 1851. 



For the development, see Rathke's Monograph, ' Ueber die Bildung 

 und Entwickelung des Flusskrebses,' 1829; and for the de- 



