248 



ON THE BLACK PULP LEECH. 



cessful experiments were made to discover its food, which seems chiefly 

 derived from the under surface of aquatic plants in vigorous vegetation ; 

 and animal substances are likewise voraciously consumed. 



The sexual union takes place with the black planaria, though rarely to 

 be witnessed ; nor had I completely conviction of the fact before August, 

 1812, notwithstanding preceding incidents had led to such an inference. 

 Long anterior to that periods I knew that this species propagated by 

 eggs, which, for the most part, are laid by those of larger size, and 

 usually in the course of autumn ; but they may occasionally be found 

 throughout the year, or obtained in winter by the more genial tempera- 

 ture of an apartment. No external characteristic distinguishes the male 

 and female planaria, if there be actually a difference of sex between 

 them : that which is represented here, produced an egg, which entitles 

 it at least to the character of female ; and what I supposed the male, 

 was probably not quite so large. The egg is imbued by a yellowish 

 viscous matter, attaching it to any twig or straw selected for the deposit, 

 or the side of the including vessel. It is of a perfect oval figure, of a 

 dark-brown or chocolate colour, provided with a hard shell, and in every 

 respect resembles a bird's egg in miniature. Those of the largest 

 size, for great inequalities prevail, are about a tenth of an inch long ; 

 and one of this description is seen, somewhat magnified. I cannot 

 confidently affirm that more than a single egg is produced by each 

 planaria, but each egg contains several young, of the palest grey, or 

 almost white: their anatomical structure is then best disclosed, as they 

 speedily darken by succeeding increment. Some, hatched in the second 

 week of September, were deep grey on the fourth of October. The 

 speck or eyes may be recognised at an early stage by a magnifier, Fig. 

 c, which also exposes slight discrepancies in shape between the young 

 and the adult animal. A considerable interval elapses before exclusion 

 of the young planaria ; but observations on the period seldom coincide. 

 By the most recent, four were detected in the very act of issuing from 

 the egg, on the eleventh of April, twenty-five days after it was laid : 

 all lively, the marginal specks very distinct, but not of equal number. 

 An egg of the same species was at the same time hatched in twenty-one 

 days : but a fortnight later, a third of the like age was still entire. 

 Therefore, the period required for exclusion, is to be calculated at 

 twenty-one days, or longer ; and it is certainly affected by the tempera- 

 ture of the atmosphere. The oviduct and relative organs appropriated 

 for perpetuation of the species, are most likely situated in the second 



