ASTACUS FLUVIATILIS. 289 



ually enter the circular canal which communicates with the 

 vesicle. 



There is little in these structural features to suggest an 

 organ of special sensation, but much to show that the green 

 mass is a secreting organ, and that the vesicle acts (whatever 

 other purposes it may subserve) as its duct. In all proba- 

 bility the green gland is an organ of the same nature as the 

 shell gland of the Entomostraca. 



Leydig has attributed an olfactory function to certain 

 groups of delicate setae which occur on the joints of the outer 

 division of the antennule of the Crayfish. 



The most remarkable part of the muscular system of the 

 Crayfish is the great extensor muscle of the abdomen, a com- 

 plex mass of fibres which is attached in part to the endo- 

 phragms of the thorax in front, and, behind, to the sterna of 

 the abdominal somites, a large part of the cavity of which it 

 occupies. 1 



The essential parts of the reproductive organs in the male 

 and female Astacus are very similar to one another in form, 

 both ovarium and testis having the figure of a trilobed gland' 

 situated immediately behind the stomach, and below the 

 heart. Two of the lobes are applied together, and pass for- 

 ward; the other lobe is directed in the middle line back- 

 ward. The ducts take their origin, one on each side, at the 

 junction of each antero-lateral with the posterior lobe. 



In minute structure, however, the two organs differ widely. 

 Each lobe of the testis is composed of a number of small 

 caeca, in which the spermatozoa are developed, and which 

 open into a central duct. The ovarium, on the other hand, is 

 essentially a wide sac, produced into three large caeca, each 

 of which corresponds with a lobe ; and the ova are developed 

 in the epithelial lining of the sac. The efferent ducts, again, 

 have little resemblance, the oviducts being short, wide tubes 

 which open on the coxopodites of the antepenultimate thora- 

 cic appendages, while the vasa deferentia are canals as long 

 as the body, at first very narrow, but afterward widening, 

 which lie coiled up on either side of the posterior part of the 

 thoracic cavity, where their white contents make them very 

 conspicuous (Fig. 74, gn'). Eventually, they open on the 

 coxopodites of the posterior thoracic appendages. 



The spermatozoa, like those of many other Crustacea, are 



un F< £ 1 deta \ ls ' see Suckow, " Anatomisch-physiologische Untersuchunffen." 

 Milne-Edwards has described the muscles of the Lobster at length in the " His- 

 toxre naturelle des Crustaces," torn. i. 



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