ARTHROPODA. 107 



surface as a solid mass consisting of an outer layer, from which 

 the retinulse originate, and an inner layer which is converted 

 into nerve-fibres connecting the eye with the optic ganglion. 



(2) The endoderm (hypoblast) is converted into the epithelium 

 of the mid-gut, and of the digestive gland which grows out from 

 each side of this. 



(3) The mesoderm (mesoblast) does not divide into regular 

 mesoblastic somites (except in the tail at a comparatively late 

 period). Irregular spaces are formed in it which become the 

 venous sinuses. The circulatory and respiratory organs (except 

 the epithelium and cuticle), the excretory (?), and reproductive 

 organs, together with the muscular system and the connective 

 tissue uniting the various parts, all arise from this layer. 



Further Remarks on the Development of the Crayfish : 



It is a noteworthy fact that most Invertebrates which inhabit rivers 

 have a direct development, that is the young, when hatched, closely 

 resemble the adults in appearance. Marine forms, on the contrary, are 

 commonly hatched as free-swimming larvce, leading an independent exist- 

 ence for some time, and differing more or less from their parents, which 

 they ultimately come to resemble by passing through a series of changes 

 known as a metamorphosis. This is the case, for example, with crabs and 

 lobsters, and a larval form is of obvious advantage to them since it affords 

 a means of widening their area of distribution. But to most fluviatile 

 animals free-swimming larvae would be a positive disadvantage, since they 

 would be very liable to be swept down to sea by the current and so 

 perish. There is some danger of this even in the case of the young crayfish, 

 but the risk is reduced to a minimum by their attachment immediately 

 after hatching to the swimmerets of the mother, and it is stated that 

 even after this she shelters them for a time under her tail, when danger 

 threatens. 



Other Crustacea. 



The Lobster (Homarus) closely resembles the crayfish in structure, but 

 differs in the following respects: Ext. Chars. Last thoracic segment 

 completely fused with cephalothorax. Telson undivided. 1st abd. app. 

 of $ two-jointed, with spoon-shaped end ; 2nd ditto with plate-like endo- 

 podite. Well-developed swimmerets on 1st abd. segment of $ . Antenna 

 with small squame. Digestive Organs. Mid-gut with small bilobed caecum. 

 Intestine of a smooth anterior and ridged posterior part, with a dorsal 

 caecum at the junction of the two. Digestive gland extends far forwards. 

 Respy. Organs. Podobranchs arranged as in crayfish, but the gill-plume is 

 not fused with the epipodite. Arthrobranchs one less in number, the one 

 found on the second thoracic segment of the crayfish being absent here. 

 Pleurobranchs four in number. Reproductive Organs. Two tubular sper- 



