164 DESCRIPTIONS OF PREPARATIONS. 



is separate and moveable ; whereas the anterior sterna are calcified as a 

 continuous whole. 



The six abdominal somites are all free and connected one to another 

 by soft intersegmental membranes. Each somite bears attached a single 

 pair of appendages, swimmerets or pleopoda. The somite forms an un- 

 broken ring. Its ventral region between the attachments of the limbs is 

 the sternum, while the opposite dorsal area is the tergum. The flap pro- 

 jecting ventrally and laterally is the pleuron, and a small space between 

 the socket for the limbs and the base of the pleuron is known as epimeron. 

 A typical swimmeret, e. g. that of the fourth somite, consists of a basal 

 protopodite bearing two processes, an inner, the endopodite, and an outer, 

 the exopodite, both fringed with setae. There are two joints in the 

 protopodite, a small basal coxopodite and a larger basipodite. The endo- 

 podite consists of a simple unjointed basal and a terminal jointed or 

 annulated portion. The exopodite is similar but its parts are smaller. 

 The last pair, often termed par excellence the swimmerets, are somewhat 

 modified. The protopodite consists of a single joint The endopodite and 

 exopodite are expanded into broad thin plates, and the latter is divided by 

 a transverse joint. This pair of limbs together with the telson make up the 

 caudal fin, by means of which the animal deals a powerful stroke upon the 

 water, and darts backwards whenever the tail is suddenly flexed. The first 

 pair of abdominal limbs is either wanting altogether in the female, or one 

 limb is present without the other. It is rare for both to be present. When 

 present they are reduced to slender filaments with a minute basal joint or 

 protopodite, and a jointed terminal portion which perhaps represents an 

 endopodite. The two first pairs of these appendages are modified in the 

 male for sexual purposes, and are to be seen in the two following pre- 

 parations. 



The English and Irish Crayfish is widely spread over the Continent. 

 It constitutes the variety of Astacus flnviatilis known as A. torrentium, the 

 crevisse a pieds blancs. There is a Crayfish which is found also widely 

 spread over the Continent and which resembles A. torrentium very closely. 

 This constitutes the variety of A. fluviatilis known as A. nobilis> the 

 Iicrevisse a pieds rouges. 



The body walls or integument consist of (i)^uticular structures for the most 

 part calcined ; of (2) a single layer of ectoderm or hypoderm cells (=chitinogenous 

 cells); and of (3) connective tissue imbedding pigment cells, bloodvessels and 

 nerves. 



The cuticular structures differ somewhat in the hard calcified and the soft 

 intersegmental regions. The former is composed of four distinct strata : (i) a 

 cuticle, (2) a pigmented calcified layer, (3) a non-pigmented thick layer containing 

 a very large proportion of calcareous matter, and (4) a non-calcified softish and 



