76 LECTURE IV. 



found in a higher grade of development, or with sexual organs, in 

 the intestines or other parts of the slug. Siebold therefore concludes 

 that they may be restored to their native locality — the intestinal 

 canal of a warm-blooded animal — by the slug being devoured by some 

 mammal or bird, and that there they undergo Iheir further and com- 

 plete development ; quitting their cysts, and forming their segments 

 with the generative organs, which are detached by spontaneous 

 fission. 



This seems a bold hypothesis, and it would be a hazardous one if 

 it rested on the mere facts of the resemblance of the vermicule (yfig. 

 33.) in the pulmonic cysts of the slug to the embryo in the ovum of 

 the Cestoidea. But the modifications of the hooks and suckers in 

 some other vermicules, having the general character of cestoid 

 larvas, correspond so closely with peculiarities of the same parts in 

 the tapeworms of animals feeding on those in which such larvae are 

 found encysted, as to add greatly to the probability of the migratory 

 hypothesis. Siebold discovered vermicules, having the general cha- 

 34 racter of cestoid larva3, in cysts in the coats of the 



^r'v^g^^l intestine, and free in the cavity of the intestine, of a 

 j^ ---^^^ ") cuttle fish {Eledone moschatd). These vermicules had 

 ^-^ _ ,^ ^ a large quadrangular head, divided by a slight con- 

 striction from the body, and bearing on its anterior 

 flattened surface nine suckers, arranged as VLifiQ' 35., 

 the largest in the middle, the rest in four pairs, with 

 the inner one largest in each. The parenchyme of 

 the body of this vermicule presented the clear calcareous corpuscles 

 characteristic of the cestoid tissue, together with the 

 four longitudinal canals, as in Jig. 33, I; with which 

 evidence of its cestoid character, the next step was 

 to examine what known species of mature or sexual 

 cestoid presented the nearest resemblance to the pecu- 

 liar cephalic organisation of the presumed larva. This Siebold 

 found in the Bothriocephalus auriculatus, in which the four angles 

 of the head are produced into distinct lobes, each bearing on its 

 flattened anterior surface a pair of suckers corresponding in their 

 inequality of size and relative position with those in the larvas from 

 the cuttle-fish ; the large central sucker having disappeared in the 

 course of the modification of the central interspace through the 

 progressive development of the lobes {Jig. 34.). That such change 

 takes place in the condition of the peculiar head of the B. auriculatus 

 is made probable by further changes observed in different individuals 

 of that species. The suckers, for example, are seen in the younger 

 specimens, and gradually disappear in the older ones. The animal 



