DEGENERA TION, 385 



mites. It possesses two pairs of jointed limbs, and certain style-like 

 organs pertaining to the mouth. There is thus the clearest evidence 

 that linguatulina is a degraded animal. It is the degenerate descend- 

 ant of a free-living and apparently four-legged or it may be eight- 

 legged ancestor ; and its further history seems to afford a clew to the 

 causes of its retrogression. For the four-legged larvae of linguatulina 

 escape, while still within the ^g^^ from the nose of the dog or sheep 

 host which has harbored their parents. Received along with food into 

 the body of the hare or rabbit, the larval being liberates itself. From 

 the rabbit's digestive system it bores its way through the tissues to 

 the liver, thus reminding one strongly of the similar migrations of the 

 embryo tapeworm. In the liver further changes ensue. Frequent 

 moltings become the order of the day, and at length they assume a 

 worm-like aspect and remain thus, still imperfect, until, by transfer- 

 ence to the body of dog, wolf, or sheep, and by passage to the frontal 

 sinuses, they acquire perfection of their life-functions. If the history 

 of these beings teaches us anything concerning their past, it points to 

 a free and active state as their original condition, and to the probable 

 acquirement, first, of a lodgment in the digestive system of one ani- 

 mal as a relatively simple parasite ; and, secondly, of a further modifi- 

 cation of habit, transferring at once its perfection and comj)leted deg- 

 radation to the forehead cavities of a second host. 



But the conditions which make for the degeneracy of an animal 

 are, as we have seen in the case of the barnacles, not always associated 

 with a parasitic habit. Mere fixation, as we have observed, secures 

 the disappearance of useless organs, such as organs of motion and 

 sense-organs, which, being possessed by the young form, clearly indi- 

 cate that the ancestry of the animals in question has at any rate been 

 capable of leading to better things than the descendants represent in 

 their existent persons. The sea-squirts, or ascidians, besides serving 

 as a text for the derivation of vertebrates, and for abnormal ways in 

 the animal chemistry which imitates the plant's work, have been se- 

 lected as fruitful objects of discussion by those biologists who find in 

 the idea of degeneration an explanation of knotty points in natural 

 history. For the same voice that proclaims the fact that a sea-squirt 

 which is a mere rooted bag with a double neck (Fig. 16) begins life 

 as a free-swimming, tadpole-like larva (Fig. IT, 5), tells us in the same 

 breath that there must have been retrogression and degeneration from 

 an active condition to produce the sac-like adult state. The assertion 

 that the youthful sea-squirt, moreover, possesses first a rod-like body 

 called the notochord (Fig. 17, n) only found besides in the young of 

 vertebrate animals, is also to be taken as implying the superiority of 

 ascidian infancy to sea-squirt maturity. And, when it is added that 

 the elderly squirt wants the sense-organs and nervous cord which the 

 larva possesses, it may well be argued that sheer degeneracy of habit 

 and structure can alone account for the sweeping transformations 



TOL, XIX. 25 



