PHYLUM ARTHROPODA 



609 



-mesenf 



numerous narrow spaces between the lamina- the air penetrates, 

 and oxygenates the blood which enters the interior of the laminae 

 from the ventral sinus. 



A pair of coxed glands, situated near the base of the fifth pair 

 of appendages, are, in the embryo Scorpion, represented by tubes 

 which, like nephridia, effect a communication between the body- 

 cavity and the exterior ; in the adult Scorpion the tube assumes 

 the form of a closed gland, and its function is quite uncertain. 



The nervous system is constructed on a plan which bears a 

 considerable resemblance to that of the Crayfish and that of the 

 Cockroach. There is a bilobed 

 cerebral ganglion or brain 

 (Fig. 498, brn.) from which 

 nerves are given off to the 

 eyes ; a nerve collar formed of a 

 pair of (.mopkageal connectives 

 unites ventrally in a sub- 

 ossophageal ganglion, forming 

 the anterior part of a ventral 

 nerve cord (ne. co.). The con- 

 nectives and sub-oesophageal 

 ganglion give rise to the nerves, 

 to the first six pairs of append- 

 ages, and to the operculum, the 

 pectines, and the two following 

 segments. The first ganglion 

 in the nerve cord appears in 

 the eleventh segment (reckon- 

 ing the cephalo-thorax as made 

 up of six) ; behind which a 

 ganglion appears regularly in 

 each segment as far back as the 

 fourth of the post-abdomen. 



The organs of special 

 sense are the eyes and pectines. 

 The lateral eyes (Fig. 518) are 



similar in character to the simple eyes or ocelli of Insects. 

 The two larger central eyes (Fig. 519) differ from them in 

 having the retinal cells arranged in groups as in the com- 

 pound eye, but resemble them in the presence of a single 

 cuticular lens. 



Reproductive organs. In the male the testes consist of two 

 pairs of longitudinal tubules united by cross branches. These 

 are connected with a median vas deferens, the terminal portion of 

 which, provided with accessory glands, is modified to form a double 

 penis ; its external opening is just behind the operculum as 

 already noticed. There is an unpaired ovary, which is made up of 



mat 



-prod 



FIG. 499. Dorsal view of the internal organs 

 of Scorpion. ck<.l. chelicerse ; h<:p. liver; 

 /<>/). (7. hepatic ducts; mat. Malpighiaii 

 tubes ; mesent. mesenteron ; proct. procto- 

 damrn; sal.gld. salivary glands; stomo. stomo- 

 dieum. (From Leuckart after Blauchard.) 



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