170 CLASS V. 
recognised by their curved extremity or by the sexual organ exter- 
nally visible; this penis is in most species double, in T'ricocephalus 
and T’richosoma single. The organ which prepares the germ or 
seed has the form of a slender convoluted canal. It is single in the 
males: in the females, with few exceptions, it is double. These 
canals are of great length: according to CLoqurt, in the male 
Ascaris lumbricoides when quite unravelled it measures from 23— 
feet, and in the female each of them measures 4—5 feet. The dif- 
ferent divisions of the canal may be considered to be ovary or testis, 
and vas deferens or oviduct with uterus. The terminal portion m 
the female is distinguished as uterus by its greater circumference 
and its vigorous peristaltic motion. A very wide, longish sac- 
shaped structure at the termination of the canal in the male is to be 
considered as vesicula seminalis. The external sexual opening is, in 
the male, always situated at the posterior extremity of the body ; 
in the female ordinarily further forward, and in some species in the 
middle, or towards the anterior extremity. Non-sexual Nematoids 
are met with?; CREPLIN gives it as a general rule that a Nematoid 
living in a perfectly closed cyst, or shut up on every side by a mem- 
brane, never has sexual organs. It has been suspected that these 
species are in an incomplete state, and can attain their perfect deve- 
lopment only in other localities*. 
We here approach what, until very recently, was one of the 
most obscure problems in the economy of the Entozoa: and 
Misscuer was fully justified in his remark that many of the 
observations relating to their development are riddles of Natural 
History*. [With respect to the suctorial worms our information is 
in fact only fragmentary: yet since the observations refer to very 
different periods of their development in different Trematodes, we 
are able from analogy to collect a tolerably connected history of the 
whole process in any one of the class.]_ It is well known that the 
1 In Ascaris lumbricoides, these parts are figured in the work of CLoQuET already 
cited, Pl. 11. figs. 8—r1o, Pl. Iv. 
2 Von SrEBoLD, WinaM. Archiv f. Naturgesch. tv. 1. 1838, s. 302—312 ; CREPLIN, 
ibid. s. 273. 
3 MrrescHER, however, has observed ovaries in Filaria Pisciwm. WIEGMANN’S 
Archiv. 1841, II. 8. 301. 
4 ¥. Miescugr Beschreibung und Untersuchung des Monostoma bijugum. Basel, 
1838, 4to. s. 24. 
