ON THE BLACK PULP LEECH. 249 



ventral pore : neither this nor the first is conspicuous in all planaria ; 

 in many they are scarcely perceptible, and appear connected in some by 

 a lighter line on the abdomen. 



But, independent of propagating by eggs, the black planaria, from 

 that indestructibility of life preserving it under mutilation, is privileged 

 to multiply its species in proportion to the violence offered to its other- 

 wise delicate frame. It may almost be called immortal under the edge 

 of the knife. Innumerable sections of the body all become complete 

 and perfect animals : if the head be cut off, a new one replaces it : if 

 the tail be severed, a new tail is acquired : nay, if one half of the whole 

 a'nimal be longitudinally separated from the other, the defective portion 

 is speedily repaired. 



A grand and leading law of nature is invariably to fulfil her original 

 object ; whence we must conclude, that the performance of accidental 

 lacerations, which we occasionally witness, results from some unknown 

 condition opposing the progress of reproduction. In the artificial muti- 

 lations, where advances can be watched, the period of renewal is strictly 

 commensurate with the temperature of the atmosphere. Regeneration 

 is retarded, or altogether suspended, by the cold of winter, promoted 

 by the heat of summer, and still farther accelerated by augmenting the 

 natural warmth of the air. A number of planariae mutilated on the 

 twentieth of January, 1803, had become entire animals in the succeeding 

 April : but all the new parts were of a lighter colour, which was long 

 in approaching the sable hue of the old : and it may be questioned 

 whether they ever grow equally dark as those regularly deepened by 

 age. Thus we see a black head, with a light coloured tail ; a dark 

 body, with a white head ; and one longitudinal half as black as jet, 

 while its corresponding portion is of a clear grey. The elementary 

 parts of all animals seem colourless : their future opacity is derived only 

 from certain assimilations of extraneous substances, the atmospheric 

 influence, or supervening rigidity of the parts. Pellucidity marks the 

 rudiments of life : fishes are transparent on exclusion from the egg ; 

 insects are pale on leaving the chrysalis ; their organs are infirm and 

 their senses obtuse. But scarce have they experienced the genial effects 

 of the air, when their members expand, strength is acquired, and instinct 

 becomes active ; all as the universal shade of the body darkens. Yet 

 exposure to the atmosphere, unmixed with another fluid, is inevitable 

 destruction to the pulp leech (planaria). If, chancing to wander beyond 

 the confines of its native element, its endeavours to return be interrupted, 

 it contracts and grows distorted, a kind of gluten issues from the whole 



