ANATOMY 305 



glands, often called salivary glands. The succeeding portion in 

 the prosoma receives four or five more pairs of ducts from the 

 well-developed gastric glands. In the rapidly narrowing first 

 metasomatic segment the intestine receives one or two pairs of 

 Malpighian tubes, and thence proceeds to the anus, situated 

 ventrally in the last segment. 



The vascular system is of the usual Arachnid type, the 

 heart being a seven-chambered dorsal longitudinal vessel lying in 

 a pericardium, with which it communicates by seven pairs of 

 valvular ostia. Lankester^ has demonstrated several pairs of 

 superficial lateral veins connecting two deep-seated ventral 

 venous trunks with the pericardium. The lung-books are, so to 

 speak, pushed in to dilatations of these trunks, so that some of 

 the lateral veins carry blood newly aerated by the lung-books 

 directly to the pericardium. 



The nervous system is not greatly concentrated except in 

 the prosoma, where there is a single ganglionic mass which 

 innervates not only the whole prosoma but the mesosoma as far 

 as the first and sometimes the second pair of lung-books. There 

 are two mesosomatic ganglia, variously situated in different 

 genera, and each metasomatic segment has its ganglion. 



The generative organs are more or less embedded in the 

 gastric glands. There are two testes, each composed of a pair 

 of intercommunicating tubules, and connected by a common vas 

 deferens with the generative aperture, which is furnished with 

 a double protrusible intromittent organ. A pair of vesiculae 

 seminales and a pair of accessory glands are also present. The 

 female possesses a single ovary, consisting of a median and tw^o 

 lateral tubules, all connected by cross branches. 



In addition to the external sclerites a free internal skeletal 

 plate, situated in the prosoma between the alimentary canal and 

 the nerve-cord, furnishes convenient fulcra for muscular attach- 

 ment. It is known as the " endosternite." 



Brauer''^ has made the most complete study of the develop- 

 ment of Scorpio, and two of the most interesting of his conclusions 

 may be mentioned here. He has shown the lung-books to be 

 derived from gills borne on mesosomatic appendages. Moreover 

 he found in the embryo five pairs of segmental ducts — in 



1 Tr. Zool. Soc. xi. part x., 1885, p. 373. 

 ^ Zeitschr. wiss. Zool. lix., 1895, p. 351. 

 VOL. IV X 



