L obster- — A iiatoniy. Sexes. \ 9 



kidneys of the Vertebrate animals) is represented by a pair of Wall- 

 glands known as the " green glands " lying at the sides of the head ^'^^^s 



. o o ^ o Nos 1-3 



and opening to the exterior each on a small tubercle on the first 



segment of the antenna. 



The central nervous system consists of a " brain," lying in front 

 of the head, connected by a pair of cords which pass on either 

 side of the gullet with the " ventral nerve chain " in which may 

 be distinguished twelve nerve centres or ganglia. 



The eyes, as already mentioned, are set on movable stalks. 

 The black, kidney-shaped area at the end of the stalk can be seen, 

 under a magnifying lens, to be divided into numerous minute 

 facets (some 13,500 in number), for the most part square in 

 outline. It is not correct to state, as is sometimes done, that each 

 facet corresponds to a separate eye, forming a separate image of 

 the object looked at ; the whole assemblage of facets and the 

 structures underlying them co-operate to form a single image on 

 the receptive nerve-endings in the interior of the eye. 



In the basal segment of the antennule is the so-called auditory 

 organ, a small pouch open to the exterior and containing in its 

 cavity a number of grains of sand. This pouch, which has on its 

 inner surface numerous feathered hairs connected with a large 

 nerve, was formerly regarded as the Lobster's ear. Although it is 

 not impossible that it may have to do with the sense of hearing, 

 recent investigations have shown that its principal function is 

 connected with maintaining the equilibrium of the body in walking 

 or swimming. 



The dissection exhibited (see Fig. 4) is one of a male Lobster, 

 and the testis can be seen lying below the heart and giving off a 

 duct, the vas deferens, which opens to the exterior on the coxa of 

 the last pair of legs. 



Differences between the sexes. — Two preparations are 

 exhibited in order to show the chief external differences between 

 the sexes of the Lobster (Fig. 5). The most easily noticeable 

 differences are the greater breadth of the abdomen and the larger 

 size of its side-plates in the female than the male. The first pair 

 of swimmerets (which, unlike the other pairs have only one branch 

 in both sexes) are very slender in the female, but are much larger 

 and peculiarly shaped in the male. The second pair have an 

 additional lobe on the inner side of the endopodite in the male. 

 The openings of the genital ducts can be seen on the first segment 

 (coxa) of the last pair of w^alking legs in the male, and on that of 

 the last pair but two in the female. Finally, the female has on 



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