OVUM. 



The instance already referred to, of the iden- 

 tity of the Cysticercus of the liver of the mouse 

 and rat with the Taenia crassicollis of the cat, 

 and a variety of detached observations which 

 prove that the Bothriocephalus and Tctra- 

 rhynchus pass through similar changes from 

 a small Echinococcus-like animalcule to the 

 developed cestoid form, lead to the corro- 

 boration of the same general view that the 

 encysted condition of these Entozoa is an 

 incomplete non-sexual embryo or larva, from 

 which, when it passes into the free state, 

 there is formed by a process of transverse 



Fig. 27. 



B 



Tamia solium. (From Blanchard.) 



A, one of the longer mature posterior segments 

 with the sexual organs fully developed ; o, o, rami- 

 fied ovary full of ova ; o', the oviduct ; t, the tubu- 

 lar testis ; t', the penis, &c. 



B, head, neck and anterior recently formed seg- 

 ments. 



fission a segmented individual or compound 

 animal, in which each segment, as it arrives 

 at maturity, attains to sexual completeness. 

 In this process the new segments are always 

 developed between the head and those already 

 formed. If the character of sexual complete- 

 ness is to be taken as the distinguishing mark 

 of individuality, each segment of the Cestoid 

 may be looked upon as a distinct animal, 

 and the separation of them by transverse 

 fission may be compared to the separation of 

 Medusa individuals from the Strobila polype 

 stock. The Cestoid Entozoa might in the 

 same manner be considered as subject to a 

 peculiar process of alternate generation. 



In the preceding sketch of the nature of 

 the reproductive process in the Cestoid Ento- 

 zoa, I have followed chiefly the views of V. 



Siebold as explained in the interesting Me- 

 moir already referred to. It is right to state, 

 however, that the phenomena have been 

 viewed in a different light by several observers 

 of high authority. Thus, Blanchard and Van 

 Beneden consider the first stage of the Tetra- 

 rhynchus-embryo to be a Scolex, in which, 

 after it has been encysted, the Tetrarhynchus 

 is formed : this, according to Blanchard, is its 

 complete condition ; but, according to Van 

 Beneden, the so-called Tetrarhynchus is con- 

 verted into a Rhynchobothrius, and this is in 

 the last place changed into a separate Tre- 

 matode animal.* Dujardin had previously 

 taken the same view as applied to the separate 

 and independent nature of the joints of the 

 Tccnia, which he regarded as individual Trc- 

 matode animals, and described under the name 

 of Proglottis(seejtfg. 2G.X.-.)f; but though there 

 may be some points of analogy between the 

 single segments of Ta3iiia and a Trematode, 

 yet the absence of head, differences in the 

 alimentary canals, and other circumstances, 

 render the correctness of this view, at all 

 events, still doubtful. 



Trematoda. These animals, the most 

 common of which are known as Flukes (ex- 

 cluding the Planariae), comprehend a set 

 of internal parasites of a structure bearing 

 some resemblance to the Cestoidea, but 

 single, that is, not jointed or segmented. The 

 nervous and vascular systems attain to a 

 considerable degree of development : the 

 alimentary canal, which has a mouth but no 

 anus, is in some bifurcated, and in others 

 more or less ramified. The male and female 

 generative organs are united in one individual, 

 and pervade a large portion of the body of 

 the adult animal. 



The facts which have been ascertained in 

 recent times concerning the generation of 

 some of the Trematoda constitute one of the 

 most remarkable parts of the history of this 

 process among the Invertebrata. Their ge- 

 neral result may be shortly stated thus: the 

 fully grown and sexual Trematode animal, as 

 observed chiefly in the Distomata, produces 

 ova, which may pass through the earlier 

 stages of their development either in the 

 viviparous or oviparous mode, more fre- 

 quently the latter. Each of these ova has 

 formed from it an embryo in which no re- 

 semblance to the Trematode parent is to be 

 recognised, but presenting the simple struc- 

 ture of a ciliated animalcule like a polygastric 

 infusorian or a Gregarina. This embryo is 



* Bull, de 1'Acad. Roy. de Belgique, 1849, No. ]., 

 and Ann. des Scien. Nat. vol. xi. 18-49, p. 13. ; also a 

 work by the same author on the Entozoa, Brussels, 

 1850, of which I have only seen an extract in a letter 

 addressed to Milne-Edwards, in the Ann. dcs Scien. 

 Nat, 1851. torn xv. p. 309. 



t Hist. Nat. des Helminthes, 1845. 



j See also Leblond, in Ann. des Scien. Nat. 1836, 

 and Mieschcr, Bericht Naturforsch. Gesellsch. Basle, 

 1840 ; the Works of Kudolphi on Entozoa; the 

 Article ENTOZOA in this Cyclopasdia, by Owen ; 

 Kolliker's Memoir on the Development of Inverte- 

 brate Animals, in Miiller's Archiv, 1843; Eschricht 

 on Bothriocephali, 1810, &c. &c. 



