COMMON CRAYFISH. 183 



Hum in possessing three instead of two rudimentary pleurobranchiae. The Crayfishes 

 of the S. Hemisphere, with the exception of Astacoides from Madagascar, have four 

 functional pleurobranchiae. It is remarkable that these organs are entirely absent 

 in Cambarus (the only other genus besides Astacus of Potamobi'idae\ which is dis- 

 tributed East of the Rocky Mountains from the Great Lakes to Guatemala, and 

 is found also in Cuba. 



Inasmuch as the branchial filaments are all cylindrical in the Crayfishes, the 

 branchial plumes are tricho-branchiae. The Crayfishes in this respect agree with all 

 Decapoda Macrura except the genera Gebia and Callianissa, the Prawns, Shrimps, 

 and Mysidae. The branchiae of the last-named are either absent or rudimentary. 

 In the other Macrurans mentioned, as in the Hermit Crabs and Brachyura, the 

 filaments are replaced by lamellae, and the branchiae are phyllo-branchiae. 



Claus has pointed out (Arb. Zool. Inst. Wien, vi. 1885, pp. 39-47) (i) that the 

 relation of the coxopodite to the body-walls is by no means a constant one in 

 Crustacea, and the arthrodial membrane has limits often ill-defined ; (2) that in the 

 branchiferous Schizopoda (Euphausia, &c.) the branchiae, which are all trifid, form a 

 row of podobranchiae ; (3) that in the larva of Penaeus there are three rows of 

 branchial rudiments, which he terms distal, middle and proximal. The last is a 

 double row, but one of the elements appears at a later period than the other. 

 The distal rudiment on each limb represents the podobranch + epipodite(= lamina) : 

 the middle and the first proximal, the anterior and posterior arthrobranchs, and 

 the second proximal the pleurobranch, in Astacus. It may be noted that the primitive 

 distal rudiment from the second maxilliped to the third thoracic foot inclusive pushes 

 out a basal bud. The bud becomes a branchia, afterwards lost on all the feet 

 save the second maxilliped, while the primitive rudiment forms an epipodite. The 

 three sets of rudiments which first appear belong distinctly to the original basal 

 joint of the limb, but the arthrodial membrane develops in such a manner that they 

 come to lie subsequently on the coxopodite, the arthrodial membrane, and the 

 epimera. 



The cavity of the branchial stem is divided by a septum into an outer and 

 inner channel communicating at the apex. The latter is continuous with one of the 

 six blood passages or ' branchial veins ' which open into the pericardial sinus with 

 widened mouths. The former is continuous with bloodvessels coming from the 

 sternal sinus lodged in the sternal canal. Each branchial filament is similarly 

 divided by a septum incomplete at the apex. According to Haeckel the blood- 

 spaces are intercellular spaces, or lacunae, and not true vessels. The tissue is 

 spongy. The cells are pyriform, one end pointed and united to the cuticula, the 

 other swollen and nucleated, and attached to other cells (cf. Haeckel, Arch. Anat. u. 

 Phys. 1857, p. 554). 



The oesophagus and stomach (=stomodaeum), and the intestine (=procto- 

 daeum) are lined by a chitinous coat. This coat consists of a superficial delicate 

 cuticle similar to that of the carapace, and a deeper lamellate layer, sometimes pene- 

 trated by pores, especially where it is much thickened. There are numerous setae 

 in the stomach, principally in its pyloric portion, and in the intestine. They are of 

 two kinds, hollow hairs, similar to those of the carapace, very plentiful in the pyloric 

 portion, and solid continuous processes. Minute ridges secreted by the chitino- 

 genous cells, and corresponding three or more to a single cell, occur in the intestine 



