OVUM. 



25 



process of generation in which is not fully 

 understood,) nor any intermediate process of 

 gemmation. A few of them, however, ap- 

 pear to become encysted in the parenchyma 

 of organs in their young or undeveloped 

 condition, and some in a form different from 

 the parent, as in the Trichina of the muscles, 

 the so called Filaria of the peritoneal cavity 

 of fishes, and the Vibrio tritici. These en- 

 cysted Nematoidea have not been observed to 

 be possessed of sexual organs*, and they 

 are not known to be multiplied by gemma- 

 tion ; it is probable, therefore that, to attain 

 the place of their full development, they must 

 be subject to migrations from one animal to 

 another, either directly or in other ways, as 

 through water and vegetables. The ova of 

 these animals appear to possess a remarkable 

 tenacity of lite, as exhibited by their long and 

 obstinate resistance to the noxious effects of 

 external agents, -f- 



The Cystic, Cestoid, and Trematode orders 

 of the Entozoa present a more varied process 

 of ceneration, the investigation of which has 

 of late years attracted considerable attention, 

 and which has led to most interesting results 

 as to the nature and relations of several forms 

 of these animals, which were previously re- 

 garded as of a most anomalous kind. The 

 Cestoid and Trematode Entozoa have long 

 been known to possess the sexual organs in 

 the hermaphrodite arrangement, and to pro- 

 duce fecundated ova ; while the Cystic En- 

 tozoa have been observed to multiply only 

 without sexual organs, and by a process 

 analogous to gemmation, and their first origin 

 has been till lately involved in the deepest 

 obscurity. We shall presently see that many, 

 if not the whole of them, may be either un- 

 developed or metamorphosed aberrant forms 

 of cestoid or trematode animals. J 



This view appears first to have been sug- 

 gested by Steenstrup, in connection with his 

 researches on alternate generations ; and it 



* See a Memoir by V. Siebold, on the Nonsexual 

 Nematoidea, in Wiegmann's Archiv, 1838. 



t Dr. Henry Nelson and I have observed the de- 

 velopment of the ova in Ascaris mystax to proceed 

 for several days, while the parent bodies containing 

 them were immersed in oil of turpentine. 



J For a notice of the generation of the minute 

 parasitic animalcule called Gregarina, see the^ pre- 

 vious account of the reproduction of Infusoria,^ 



See Ray Society's Translation, 1845, p. 100. 

 " It is not unlikely," says Steenstrup, " that in 

 course of time, it may happen with them (Cystic 

 Entozoa), as it has with the whole division of the 

 asexual Trematoda of Siebold, viz. Cercaria, &c., 

 that they must be rejected from the system as being 

 earlier forms of development, or earlier generations 

 of other animals." V. Siebold remarks in a note at 

 p. 157, of his Lehrbuch der Vergleich. Anat. part i. 

 published in 1845, " Here the doubt arises whether 

 the asexual Cystica really deserve to be considered 

 as independent animals. It is very probable that 

 the vesicular worms are undeveloped Cestoids," &c. 

 See also note at p. 111. Von Siebold has developed 

 these views more fully in a recent Mem. in the 

 Zeitsch. fur Wissensch. Zool. July, 1850, translated 

 in the Ann. des Scien. Nat. vol. xv. 1851, p. 1/7; 

 and in the article Parasites, in Wagner's Handwor- 

 terbuch der Physiologic. E. Blanchard in his Keen. 



has since been adopted, in somewhat dif- 

 ferent forms, by V. Siebold, Blanchard, Du- 

 jardin, and Van Beneden, and rendered 

 extremely probable by the researches of these 

 and some other observers. Previous to 

 the adoption of this view, helminthologists, 

 looking upon the Cystic Entozoa as dis- 

 tinct and independent animals, were at a loss 

 whether to regard them as ascertained excep- 

 tions to the sexual mode of propagation, or to 

 continue to prosecute their inquiries in the 

 hope of being able to discover a process of 

 generation in them analogous to that prevail- 

 ing in the greater majority of the animal 

 kingdom ; and many were thus misled into the 

 error of searching for ova where none existed or 

 were required. Thus Gulliver erroneously de- 

 scribed certain calcareous particles in the mem- 

 brane of Cysticercus as the ova of the animal*, 

 and H. D. Goodsir, in his instructive paper on 

 the production of the young in that animal, 

 and in the other forms of Cystic Entozoa f, 

 failed to distinguish between that which might 

 be merely a process of gemmation and the 

 origin of the embryoes from true ova.J 



Cystic Entozoa. The Cystic Entozoa pre- 

 sent themselves in three principal forms, viz. 

 Acephalocyst, Caenurus, and Cysticercus. The 

 two first are usually found as compound or 

 aggregated animals; the last is more fre- 

 quently seen in the single or isolated condition. 



Some of the vesicular hydatid tumours, 

 constituting the various kinds of so called 

 acephalocysts, have long been known to con- 

 tain small Echinococci floating in the fluid of 

 their interior. Repeated observations have 

 demonstrated the existence of these animals 

 in the acephalocysts ; and it seems very pro- 

 bable that, in the end, it will be necessary to 

 withdraw the distinctions between the various 

 kinds of these cysts, as they will all, by suffi- 

 ciently accurate observation, be found, at 

 some period of their growth, to contain in a 

 more or less complete condition, the small 

 animals of Echinococci, or their remains. 



The Echinococci are produced by non- 

 sexual generation, or by gemmation from the 

 membrane of the vesicle, probably from the 

 middle or germinal membrane, as it has been 



sur 1'Organis. des Vers, in Ann. des Scien. Nat. 1847 

 vol. vii. p. 120. excludes entirely the Cystica from a 

 separate place in the systematic arrangement, bring- 

 ing them under Cestbidea, and affirms decidedly 

 that the distinction between them ought now to 

 cease, as they are shown to be different states of the 

 same animals. He refers to De Blainville as having 

 previously entertained the same view. See also 

 Dujardin, in Annal. des Scien. Nat. for 1813, and 

 Hist. Nat. des Helminthes, 1845; Miescher, Be- 

 richt iib. die Vcrhand. der Naturforsch. Gesellsch. in 

 Basel, 1840; and Van Beneden, Ann. des Scien. 

 Nat. 1851, p. 309 ; and a work on the Entozoa, pub- 

 lished at Brussels, in 1850, which I have not seen. 



* Med. Chir. Trans, of Lond. vol. xxiv. 1841. 



f Trans. Roy. Soc. Edin. vol. xv. 1844, and in 

 Anat. and Path. Observations, 1845. 



j See also Rose, in Med. Chir. Trans, vol. xxxi. 

 1848. 



8 See V. Siebold's Report on Zoology, in Ray 

 Society's publications, for 1845 and 1847 ; also Bur- 

 dach's Physiol. B. ii. 



