380 



AEACHNIDA. 



Fig. 191. 



parts of the body. The extent to which centralization is here carried 

 will be at once appreciated by reference to the annexed figure (fig. 190) : 

 the encephalic masses (a a), whence the optic nerves distributed to the 

 ocelli are derived, are in close contact with the anterior part of a large 

 ganglion (c), that represents all the abdominal ganglia collected into one 

 mass ; and from the posterior part of this, nerves (n n), destined to supply 

 the parts contained in the abdomen, derive their origin. The thoracic 

 ganglia (ee) are fusiform, and placed on each side of the mass (c), with 

 which they are apparently amalgamated at one extremity, while from 

 the opposite they give off the nerves appropriated to the legs. 



(975.) The ocelli or eyes of Arachnidans have been minutely investi- 

 gated by Miiller*, and seem to present a type of structure very far supe- 

 rior to that of insects. In the Scorpion, this distinguished anatomist 

 succeeded in detecting most of the parts which enter into the construc- 

 tion of the eye of a vertebrate animal, and, moreover, a great similarity 

 in their arrangement. The cornea, a globular lens, the aqueous and 

 vitreous humours, the retina and choroid, were all found nearly in their 

 usual relative positions ; so that the sense of vision in these animals 

 must be extremely perfect. 



(976.) The sexual organs of the male and 

 female Arachnidans exhibit very great simplicity 

 in their structure. The testes, or secreting ves- 

 sels, of the male Spider are two long caeca 

 (fig. 191, b), lodged in the abdomen, and termi- 

 nating by simple orifices at the ventral surface. 

 No external intromittent organ is perceptible ; 

 and it was on this account that the peculiar 

 apparatus above referred to, situated at the ex- 

 tremity of the maxillary palpus, was so long con- 

 sidered as giving passage to the impregnating 

 secretion. The singular instrument already de- 

 scribed ( 957) would seem, indeed, to "be in 

 some manner really siibservient to the fecundating 

 process, being used most probably as an exciting- 

 agent preparatory to the intercourse between the 

 sexes. 



(977.) The ovigerous system of the female is equally devoid of com- 

 plication, and, like the male testes, consists of two elongated membra- 

 nous sacculi, in which the eggs are formed and brought to maturity. 

 The impregnation of the ova is evidently effected by the simple juxta- 

 position of the external orifices of the two sexes : yet such is the ferocity 

 of the female spider, that the accomplishment of this is by no means 

 without risk to her paramour ; for the former, being far superior to the 

 male both in size and strength (fig. 192, A, B), would infallibly devour 

 * Ann. des Sci. Nat. torn. xvii. 



Generative organs of 

 male Spider: a a, pul- 

 monibranchiae ; b, testes ; 

 c, cephalothorax. 



