28 



OVUM. 



brought under the genus Tetrarhynchus. In 

 fact, this kind of animal undergoes such re- 

 markable changes in its transition from its 

 first simple Echinococcus-like encysted form 

 to its free segmented sexual Ttenia-like shape, 

 that it is not wonderful that its history should 

 have been obscure, and that great doubts 

 should still prevail with someHelminthologists 

 as to its origin, development, and zoological 

 relations.* 



It has already been observed, that none of 

 these three kinds of Cestoid Entozoa attain 

 to sexual completeness while they are en- 

 cysted ; and it seems probable that they are all 

 subject, more or less, to migration, in order to 

 gain their free habitation in the alimentary 

 canal of animals, where their segments ac- 

 quire the male and female generative organs. 

 The fecundated ova, produced in enormous 

 numbers from each segment, do not in general, 

 so far as is known, become developed into 

 embryoes in the intestine of the animal in- 

 habited by the Cestoid, but are evacuated 

 along with the faeces, either separately after 

 being discharged from the oviducts of the 

 Cestoid, or before their discharge by the 

 disjunction of the more ripe terminal segments 

 from the rest of the animal. The migrations 

 to which the ova and young of the Taenioid 

 animals are thus made subject have hitherto 

 opposed so great an obstacle to the observa- 

 tion of their development, that we are as yet 

 in possession of very few continued series of 

 observations in which the whole progress of 

 development from the ovum to the complete 

 segmented animal has been traced. Some 

 important contributions of this kind have, 

 however, recently been made, and the great 

 modifications which the views of comparative 

 embryologists have undergone, from the novel 

 and various aspects in which many of the 

 phenomena of development are to be regarded 

 in instances of alternate generations, have 

 already indicated paths of inquiry by which 

 this very curious and intricate history may 

 ere long be completely unravelled. The ac- 

 companying figures from Dujardin's work 

 show the progress of formation of a small 

 Taenia inhabiting the Shrew, and give a suf- 

 ficiently good idea of the nature of this pro- 

 cess in a Taenia, which consists of compara- 

 tively few segments (fig. 96. a to ?'.). 



Von Siebold has traced with care a part 

 of the process of development of a minute 

 Cestoid inhabiting the pulmonary sac of the 

 red snail (Arion einpiricorum) in the encysted 

 condition. Into this situation the minute 

 Taenias are introduced from the exterior : they 

 consist of the head with its double circlet 

 of ten hooks each, and four suckers, and a 

 body which is at first entirely destitute of 

 segments, not longer than the head, and form- 

 ing a soft vesicle, within which (as in other 



* Von Siebold proposes to substitute the genus 

 Tetrarhynehus for the following five genera distin- 

 guished by Dujardin, viz., Ithynchobothrius, Antho- 

 cephalus, Tetrarhynehus, Gymnorhynchus, and Di- 

 bothriorhynchus. Zeitsch. f. Wiss. Zool. 1850, and 

 Arm. des Sc. Nat. 1851. 



Cystic Entozoa previously mentioned) the 

 head is retracted, so as to give the whole a 

 globular shape. V. Siebold regards it as 

 nearly certain that these minute Taeniae only 

 attain to their segmented and complete sexual 

 condition when they have been located in the 

 alimentary canal of Vertebrata (Birds and 

 others) preying upon the snails in which the 

 younger forms of the Taeniae reside. 



Fig. 26. 



Development of Tamia pistillum of the Shrew. 

 (From Dujardin.*) 



a, embryo within the ovum, just about to quit it, 

 with three pairs of hooks ; b, embryo that has left 

 the ovum, the hooks capable of rapid and exten- 

 sive movements ; c, embryo moving freely (of the 

 Trenia serpentulum of the magpie) ; d, e, very young 

 embryoes of Taniia pistillum ; f, y, h, i, different 

 stages of growth of this Tajnia ; the separation of 

 the segments gradually increasing, and the develop- 

 ment of the reproductive organs in the posterior 

 ones ; k (more magnified), the proglottis, or free 

 moving separated segment of this Tasnia. 



