170 CLASS V. 



recognised by their curved extremity or by the sexual organ exter- 

 nally visible ; this penis is in most species double, in Tricocephalus 

 and Trichosoma 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 CLOQUET, in the male 

 Ascaris lumbrico'ides when quite unravelled it measures from 2^ 3 

 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 in 

 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 1 . Non-sexual Nematoi'ds 

 are met with 2 ; 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 3 . 



We here approach what, until very recently, was one of the 

 most obscure problems in the economy of the Entozoa : and 

 MIESCHER was fully justified in his remark that many of the 

 observations relating to their development are riddles of Natural 

 History 4 . [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 lumbrico'ides, these parts are figured in the work of CLOQUET already 

 cited, PI. n. figs. 8io, PI. iv. 



2 VON SIEBOLD, WIEGM. Archiv /. Naturgesch.iv. i. 1838, s. 302312 ; CREPLIN, 

 ibid. s. 273. 



3 MIESCHER, however, has observed ovaries in Filaria Piscium. WIEGMANN'S 

 Archiv. 1841, n. s. 301. 



4 F. MIESCHER Beschreibuny und Untcrsnchung des Monostoma bijityuni. Basel, 

 1838, 4to. s. 24. 



