COMMON CRAYFISH. 165 



thin layer. The organic substratum is chitinoid. The salts are Calcium carbonate 

 and phosphate, chiefly the former. 



(1) The cuticle is structureless, resistent, and of a yellowish tint. It shows 

 areae corresponding to the outlines of the ectoderm cells, and inside these areae 

 slight ridges. The latter are supposed by Braun to be due to the coalescence with 

 the young cuticle during its formation of processes similar to those of the intestinal 

 cuticle. Both areae and ridges are more distinct in the newly formed cuticle. 



(2) The pigmented layer consists of a system of fine lamellae, parallel to the 

 surface, and alternately more and less refractile, the latter being mere lines. The 

 lamellae are perforated by minute close-set vertical pores. The pigment lies 

 between the more refractile lamellae in the form of bluish-black-granules, sometimes 

 furnished with processes. 



(3) The third layer makes up the chief substance of the hard parts. Its 

 refractile lamellae are thick, well calcified, and perforated by pores continuous with 

 those of the pigmented layer. The lamellae become thinner towards the inner 

 surface, where they merge insensibly into the fourth layer. 



(4) This layer is very distinct in the Lobster. It is not recognised by Braun, 

 but according to Vitzou it is composed of delicate non-calcified lamellae, and the 

 innermost show impressions or outlines corresponding to the ectoderm cells. It is 

 said to possess but few vertical pores. 



There are certain variations in different regions from this typical structure. 

 The knobs on the opposing surfaces of the chelae, certain spines, e. g. those on the 

 posterior dorsal edge of the basal joint of the exopodite in the last pair of 

 swimmerets, are colourless. The pigmented layer is absent, and the cuticle is 

 greatly thickened in these places. In the articular and intersegmental membranes 

 the cuticle is present, but the remaining layers are represented by non-calcified 

 lamellae not distinguishable into systems. The inner wall of the branchiostegite 

 is extremely thin, and is composed of a membrane resembling the cuticle. Areae 

 are absent here as they are on the eye-stalk, and in the oesophagus and stomach. 

 They are very well marked in the intestine, and each area supports 3-6 pointed 

 ridges. The cuticular structures of the alimentary canal are composed of a cuticle 

 and underlying lamellae, which are thickened and calcified only in the gastric 

 ossicles. Vertical pores are found also only in the same structures. There are 

 integumental setae or hairs and glands. The setae are either hollow or solid. The 

 hooked setae on the inner wall of the branchiostegite are solid, and certain solid 

 setae in the oesophagus and stomach are probably simply processes of cuticle. 

 Hollow setae are either closed or open at the base. The former are most 

 numerous, and their stem is beset with solid barbs, i. e. they are feathered. The 

 latter are chiefly confined to the antennae. They are short, thick-walled, and their 

 stems are destitute of barbs. A pore-canal pierces all the layers of the integument 

 except the cuticle, and leads to the base of each seta. The cuticle is continuous 

 with the base of the hair, which is generally lodged in a slight depression. The 

 pore-canal contains a process of the ectoderm, in which nuclei are present. It is 

 prolonged internally below the integument by the hair-tube. This structure has 

 nucleated walls, and expands at its inner extremity into a bulb in which cell-outlines 

 are distinguishable. It is about half the length of a hair, and contains a hair 

 papilla which fills its cavity. A formation of new hairs precedes the formation of a 



