360 THE AMERICAN NATURALIST. [Vou. XXXII. 
connection with the commensals of this species is the fact that the 
pigment is heavier and more uniformly distributed in them than in 
the free-living individuals. In another species, however, vtz., 
pulchra, which lives “as a common messmate (or possibly parasite) 
of two animals wide apart in the organic scale, Holothuria californica 
and Lucapina crenulata,” the wholly hidden specimens may be 
destitute of pigment. 
Polynoé gigas the author finds to be almost always asymmetrical in 
the arrangement of the elytra and dorsal cirri. Of nine specimens 
examined, only three had the same number of elytra on each side; 
and of these three, only one was fully symmetrical. W. E.R. 
Regeneration of the Earthworm’ Of late there has been 
a noticeable revival of the old interest in problems of regeneration of 
lost parts in animals, but it has been rather striking that so many 
observers have been content to use only the old methods available 
before the present era of microtome and perfected staining technique. 
In contrast to them comes the second part of the investigations of © 
Dr. K. Heschler,’ who studies by serial sections the newly forming 
heads in nearly one hundred earthworms Por which the first four or 
five segments had been cut off. 
Some of the results obtained are briefly noted below, but it should 
be kept in mind that the author does not claim to have exhausted 
the most difficult question of the histology of regenerating organs in 
the earthworm, and that he freely concedes that the interpretation of 
the confused and complex masses of tissue we find in these regen- 
erating heads has a large subjective element. 
During the first week after the removal of the head there is but 
little actual regeneration of parts. The wound heals by the forma- 
tion of a cicatrix that is made up chiefly of lymph:cells; but after a 
few hours spindle-shaped cells of undetermined origin are added to 
it. The epidermis grows over this cicatrix in a few days, while the 
intestine closes up and draws back so that the cicatricial tissue lies 
between its blind end and the new epidermis. 
After this first period there is active regeneration accompanied by 
mitotic cell division. New cells — “regeneration cells ” — wander 
into the cicatrix from epidermis, muscle, and other sources. In the 
complex mass so formed the new nervous and digestive organs of the 
head now arise. 
1 Ueber Regenerationsvorgiange bei Lumbriciden, II. /enaische Zeit. f. Natur- 
wissenschaft. März, 1898. Pls. XXI-XXVI, pp. 521-596. 
