454 



ARACHNIDA. 



the whole of the abdominal section of the alimentary canal of 

 the adult, except the rectum, and probably also the thoracic 

 section. The later history of the yolk which encloses the mesen- 

 teron has not been satisfactorily studied, though it no doubt 

 gives rise to the hepatic tubes, and probably also to the thoracic 

 diverticula of the alimentary tract. 



The general history of the alimentary tract in Scorpio is much the same 

 as in Spiders. The hypoblast, the origin of which as mentioned above is 

 somewhat uncertain, first appears on the ventral side and gradually spreads 

 so as to envelop the yolk, and form the wall of the mesenteron, from 

 which the liver is formed as a pair of lateral outgrowths. The procto- 

 daeum and stomodaeum are both short, especially the former (vide fig. 207). 



Sniiunary and general conclusions. 



The embryonic forms of Scorpio and Spiders are very 

 similar, but in spite of the general similarity of Chelifer to 

 Scorpio, the embryo of the former differs far more from that of 

 Scorpio than the latter does from Spiders. This peculiarity is 

 probably to be explained by the early period at which Chelifer 

 is hatched ; and though a more thorough investigation of this 

 interesting form is much to be desired, it does not seem probable 

 that its larva is a primitive type. 



The larvae of the Acarina with their peculiar ecdyses are to 

 be regarded as much modified larval forms. It is not however 

 easy to assign a meaning to the hexapodous stage through 

 which they generally pass. 



With reference to the segments and appendages, some inter- 

 esting points are brought out by the embryological study of 

 these forms. 



The maximum number of segments is present in the 

 Scorpion, in which nineteen segments (not including the prse- 

 oral lobes, but including the telson) are developed. Of these the 

 first twelve segments have traces of appendages, but the append- 

 ages of the six last of these (unless the pecten is an appendage) 

 atrophy. In Spiders there are indications in the embryo of 

 sixteen segments ; and in all the Arachnida, except the Acarina, 

 at the least four segments bear appendages in the embryo 

 which are without them in the adult. The morphological bear- 

 ings of this fact are obvious. 



