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ON THE BLACK PULP LEECH. 



247 



regularly darkens according to the age and increment. Probably, from 

 distension of vessels, shades of difference may prevail, which temporary 

 causes render more prominent. 



The Black Pulp Leech (Planaria) is endowed with wonderful 

 tenacity of life, a property which materially aids the researches of the 

 physiologist. This truth is forcibly demonstrated by the dreadful lace- 

 rations with which it appears in its natural element. Sometimes a 

 large section from the neck is wanting, sometimes a semicircular wound 

 almost divides the animal asunder, or one half the body has been torn 

 from the other, and still it survives the mutilation. — This was a suf- 

 ficient guide. The genus had been partitioned by nomenclators, accord- 

 ing to the number of eyes, or their total absence ; and a place from the 

 latter was assigned to the Black Planaria. It was probable, however, 

 if eyes were present, they should not be sought in the full-grown animal, 

 which might be of equal opacity to themselves ; or, in other words, they 

 would more easily be detected in regenerating parts : and, if such lace- 

 rations as I had witnessed did not destroy the animals, neither would 

 artificial separation of important organs be fatal. Several planarise 

 were therefore decapitated on the 10th of September ; and, in nearly 

 three weeks, the wounds of some being examined by a magnifier, 

 appeared not only completely healed, but a conical reproduction of the 

 defective parts protruded. The regenerated organs of all animals are 

 light and colourless ; and I thought that a row of minute marginal 

 specks, such as are usually called eyes in the vermes and molluscse, was 

 discernible. This proved no illusion, for, on the 29th of the month, 

 they were distinctly visible, of a jet black colour, seated in a pale 

 ground. Other experiments corroborated the fact ; whence, one charac- 

 teristic of the species under discussion, is a row of numerous minute 

 black specks or eyes, situate in the very margin of the anterior part. 

 The same reserve in ascribing the faculty of vision to these specks, is 

 as necessary in this instance as before ; nor can we affirm, in denying 

 them that peculiar province, that they are void of all analogy to spira- 

 cula, or organs connected with respiration. A subsequent examina- 

 tion of those planarise just mentioned as a possible variety, showed that 

 the head is sometimes so light as faintly to expose the site of the specks. 

 Thus, if the genus is to be partitioned according to the presence or 

 absence of eyes, the Black Planaria will be removed from the place 

 hitherto assigned to it. 



In the course of my earlier observations on this animal, many unsuc- 



