MECZNIKOW, ON ASCARIS NIGROVENOSA. 27 
which part of the intestine, moreover, is distinguished by its 
thickness and solidity. 
The further growth, besides the increase in size, consists 
in a further differentiation of other organs, and, above all, in 
the transformation of the sexual rudiment into perfectly de- 
veloped reproductive organs. These changes are completed 
as early as the third day of free life (in summer), at which 
time the distinction between the males and females is ob- 
vious ; and it will be found that the former are produced from 
the short-tailed individuals above described, and the females 
from the others. 
Thus it is manifest that the larve of Ascaris nizrovenosa, 
which differ in many respects from their parents, enjoy a free 
existence, in which they attain to a sexual development. 
The organization of the fully developed males of this free 
generation of A. nigrovenosa resembles, in general, very 
closely that of the above described short-tailed larvee, but 
differs from it in the further differentiation of certain organs. 
The body of the male is tolerably plump, its length being 
not more than about 14 to 1, as compared with its diameter. 
In dimensions, individuals differ considerably, some attaining 
a length of 1°] mm., whilst others are not more than 0°55 mm. 
The inwardly curved tail is tolerably thick and blunt; and 
on each side of this part may be observed a row of minute 
conical projections (Zapfen) which are connected together by 
a delicate membrane, and thence represent the similar organs 
which occur so frequently in various free and parasitic 
nematodes. 
In other respects, the external organization of the body of 
the males differs from the structure above described of the 
younger larve simply in the presence of a special excretory 
orifice in the anterior part. During the course of develop- 
ment, the intestine undergoes no particular modification, 
whilst at the same time the cells occupying the anterior part 
of the body constitute a central nervous ring. And at this 
stage also differentiated muscular fibres may be discerned. 
The reproductive organs consist of a single tract, at whose 
upper (usually curved) end the sperm-cells are visible, behind 
which lie the excessively minute spermatic corpuscles. The 
inferior portion of the sexual apparatus consists of a thick- 
walled vas deferens, which opens into the rectum. From the 
walls of the same part of the intestine arises the copulatory 
organ, consisting, in the present instance, of two connate 
spicule and an undivided chitinous sheath or groove. 
In the caudal portion of the body is lodged a mass of glan- 
dular cells, which opens on the exterior. 
