100 DEVELOPMENT OF 



whether even they probably do not attain their development 

 through a series of alternate generations, and exist at first 

 externally to the organisms with which their life is after- 

 wards inseparably connected. Although I cannot commit 

 myself to a precise reply to this query which is rather 

 beside the especial object of these pages, yet I cannot help 

 giving a few hints on the subject. The Nematoidea, 

 which in their adult state often pass from one individual 

 to another, also probably penetrate from without and as 

 embryos, the organism they infest ; they do not appear 

 to undergo a true metamorphosis, but to change their 

 skin : I am also unacquainted with any observation which 

 would justify the supposition, that there is in this group 

 any fostering of the young by precedent generations, 

 unless the genus Sphcerularia, a parasite of the Hymenop- 

 tera, which Siebold refers to this division of entozoa be a 

 " nurse' ; at least it much resembles one, appears to be 

 nearly powerless to perform spontaneous movements, and 

 contains a numerous smooth- skinned progeny, which move 

 about very actively within their parent, but bear no re- 

 semblance to it.* The cystic entozoa on the contrary, 

 betray in many ways that they are a " nwrsing" genera- 

 tion, and especially in the singular circumstance of their 

 being frequently inclosed like boxes one within the other. 

 Probably the full-grown animal of this division is quite 

 unknown, and it is not unlikely, that in course o.f time, 

 it may happen with them as it has with the whole division 

 of the " asexual " Trematoda of Siebold, viz., Cercaria, 

 Leucoc/doridium, &c., that they must be rejected from 

 the system as being earlier forms of development, or 

 earlier generations of other animals. The I]chinorhy7ichi 

 present several phenomena which are interesting with 

 regard to our object, viz., the remarkable incubation or 



* See Siebold's Abh. von d. gesclalechtsl. Nematoiden, in Wiegmann's 

 Arch. 1838, B. iv, h. 1, s. 305. Further researches will show the nature 

 of the so-called asexual Nematoidea which inhabit cases or cysts ; they inspire 

 the more iaterest since Crephn, in his remark upon Siebold's memoir, referred 

 to above (Wiegmann's Arch. 1838, B. iv, s. 373,) alleges that there is no 

 known instance of the encysted Nematoidea possessing sexual organs. 



