138 HELMINTHOZOA. 



instead of being, as Rudolph! supposed, more delicate and frail in their 

 substance than the Entozoa themselves, are defended by envelopes so 

 strong that, thus protected, they may be dispersed in prodigious num- 

 bers in various situations, and escape destruction until conveyed into a 

 nidus proper for their development. 



(358.) To form some idea of the number of ova furnished by a single 

 Tsenia of this species, it must be considered that it is furnished success- 

 ively with at least two hundred segments, which in the aggregate will 

 produce for each Taenia 25,000,000 of eggs. The mature segments are' 

 found loose in the intestine of the dog, and are able to move about with 

 considerable quickness, creeping sometimes at the rate of three inches 

 in a minute, by the contractions of which they are capable. If one of 

 these be placed in a flask, or under a moist glass bell, they will soon 

 begin to crawl about upon its surface, leaving a sort of milky track 

 wherever they pass, in which, by the aid of a lens, innumerable eggs 

 may be detected. Under these circumstances they will exist for several 

 days, until they are entirely emptied of their ova and reduced to half 

 their original bulk, when, their destiny being accomplished, they perish. 

 Therefore it cannot be doubted that, when expelled naturally from the 

 intestines of the animal in which they live, they are able to deposit their 

 ova in a similar manner. 



(359.) Many interesting facts relative to the development of the in- 

 testinal worms have been recently brought to light, and promise to lead 

 to still more important discoveries. In 1840 M. Miescher announced, 

 to a meeting of naturalists at Bale, the discovery that several genera of 

 Entozoa undergo most extraordinary metamorphoses, whereby their 

 form and character are completely changed. A Filaria, met with in a 

 fish, became changed into a flat, oval, leaf-like worm, in fact a Planaria ; 

 from the interior of the Planaria there subsequently issued a Tetra- 

 rhynchus, armed with four long proboscides; and lastly, the last- 

 mentioned form probably gave birth to a Boihriocephalus. 



(360.) Carrying out these observations, M. Van Beneden* has not 

 only confirmed the doctrine, but added very materially to our know- 

 ledge on this subject, by ascertaining that the Tetrarliynchus undergoes 

 no fewer than four distinct phases of development. 



(361 .) In the first phasis of its existence, the worm is more or less 

 vesicular in structure, being armed with four suckers, and a sort of pro- 

 boscis in the centre. It is possessed of extraordinary contractility ; and 

 in different species there are spots of black pigment, representing eyes. 

 In this condition these worms have received from helminthologists the 

 name of Scolex (Scolex polymorpTius ; Scolex Acalepharum), Sars, Tetra- 

 stoma Playfairii, Forbes and Goodsir, Dithyridium Lacertce, &c. These 

 are more especially found in the pyloric caeca. 



(362.) The second phasis is, perhaps, the most curious. In the in- 



* Ann. des Sc. Nat. 3 ser. x. p. 15. 



