30 



fiirca and pnst-fiirca are better develnped than tlie 

 corresponding- part in the prothorax. 



6. The Ye>;tral Tube or Abdominal Vesicle. 



The ventral tube is a large papilla-like organ 

 situated on the middle of the ventral aspect of the first 

 abdominal segment (fig. 3). It presents a considerable 

 range of variation, both in form and in degree of develop- 

 ment among various Collembola, but in Anurida it 

 is relatively simple in structure and remains in a 

 rudimentary condition. 



As is demonstrated by its development, the ventral 

 tube is formed by the fiision together of the pair of 

 abdominal ajipendages belonging to its segment [vide 

 text-fig. 1). It consists of a short basal column, 

 which is divided distally by a median longitudinal cleft 

 into two lobes or vesicles, and these latter are the only 

 indication in the adult of the paired origin of this organ 

 (Plate IT., fig. 3G). It is invested by the general cuticle 

 of the body- wall, but where the latter is prolonged over 

 the two terminal vesicles it becomes extremely thin and 

 flexible, its tubercles are wanting and it is quite smooth 

 (Plate II., fig. 21). In the structure of its walls it closely 

 resembles the general integument of the body ; they 

 consist of a well-developed layer of hypodermis, but 

 contain very little pigment and there is no basement 

 membrane. Where the hypodermis lines the lobes or 

 vesicles of the organ, it alters very considerably in 

 character; certain of its cells being very large, elongated 

 in a vertical direction, and tapering into a point {h.e. in 

 fig. 21). Each of these cells contains a large nucleus with 

 abundant chromatin granules. 



Arising from the terminal lobes of the organ are 

 several muscles; they pass down its central cavity and 



