250 



ON THE BLACK PULP LEECH. 



body, and death speedily ensues. Should it be hastily removed, it may 

 recover, and then the gluten comes off like a sheath, or integument, 

 investing the body ; but the transition must be immediate. Whether 

 this substance be provided by nature, to avert the baneful effects of 

 unguarded exposure, or exudes in consequence of the sufferings of the 

 animal, is uncertain. 



In the natural state, the pulp leech (planaria) probably survives 

 the cold of congelation ; yet it perished during the course of an observa- 

 tion, where the thermometer stood at 26°, and where it was imbedded 

 in a solid mass of ice. Perhaps the change was too sudden ; for numer- 

 ous societies dwell in shallow waters, affording an insecure retreat from 

 the rigour of the atmosphere ; nor are they diminished in succeeding 

 seasons. The bodies of those which perish thus, are distorted, stiff, 

 , and invested by a very thick coating of gluten. In a few hours after 

 solution of the ice, they become soft and relaxed, and are quickly 

 decomposed. 



Monstrosities sometimes occur in the black pulp leech (planaria), 

 and frequent distortions from casual injury. In 1808 one was found 

 with the tail bifid ; a ventral pore appeared in each portion, both of 

 which might be considered members of the same body. Accident may 

 give birth to superfluous parts, as well as the skill of the experimen- 

 talist ; but this species, perhaps from the energy of its reproductive 

 powers, seems less adapted for acquiring them. 



In addition to what has been said of varieties, we may observe, that 

 a few are taken in places inhabited by the black planaria, of smaller 

 size, the head rather more obtuse, and the tail suddenly drawing 

 towards a point ; whereas it ordinarily forms a regular elliptical outline, 

 unless when produced to an acute angle by extreme extension. Possi- 

 bly, however, the difference may centre in the progress of reproduction, 

 whereof the commencement has not been seen. Very minute dark or 

 light pulp leeches (planarice) sometimes dwell along with the rest, of 

 which the tail is more acute in proportion than that of the larger ones. 

 Three of these, a line and a half long, being subjected to the micro- 

 scope, traversed the glass slider with great rapidity, much exceeding 

 the usual progression of the black planaria. A scanty row of very dark 

 specks environed the anterior margin ; and the internal organisation 

 was sensibly exposed through the gelatinous texture of the body. But 

 a series of experiments would be required to entitle us to deny their 

 identity with the young of the common species ; first, because the 

 accelerated motion might be excited, from the susceptibility of a small 



