No. 3.] SOME NEW FACTS ABOUT THE HIRUDINEA. 595 
organs with the sense-organs of the annelids. If an authority 
of Professor Leydig’s standing can be presumed to stand in 
need of instruction on the differences that divide cerebral from 
peripheral sense-organs, it is not of course to be expected that 
my suggestion will meet with a less patronizing reception. 
These differences have been insisted on so often that they 
have become common property, and it indicates no small meas- 
ure of audacity in a critic to assume the role of instructor in 
regard to them. Take what are now incontestable facts in the 
phylogeny of annelid and arthropod sense-organs, and add to 
them the evidences in favor of the common derivation of the 
vertebrate organs of special sense, and is it not enough to 
awaken a very strong suspicion that the visual organs of verte- 
brates will not be able much longer to hold the position of 
isolation so long conceded to them? 
In the study of this question the following points seem to me 
of first importance: 1. Vertebrate sense-organs must be assumed 
to be derived from invertebrate sense-organs, and the history of 
the latter must furnish clues to the genesis of the former. 2. In 
the development of special senses, visual cells have made the 
widest departure from the primitive tactile cells. As the deri- 
vation of visual cells from cells of the tactile order is now, as 
I maintain, clearly established by the facts announced in this 
paper, it follows that structural and functional differences in 
sensory organs cannot be accepted as proof of diversity of 
origin. 3. The medullary plate of the vertebrate is undoubt- 
edly an enormous extension of the ancestral invertebrate plate. 
4. Sense-organs lying originally outside the neural plate have 
probably, in consequence of this extension in width, been 
brought within the medullary area. 5. The ancestral seg- 
mental sense-organs were not limited to a single pair of lateral 
lines, but to several paired lines symmetrically placed on the 
dorso-lateral and ventro-lateral surface. 
19. The caudal ganglia are seven in number, with perhaps a 
rudimentary eighth. Sometimes from ¢wo to four ganglia in 
front of the caudal group are approximated, and the group thus 
formed constitutes the anal ganglia. In several American 
species of C/epsine, however, no such group exists, all the 
lymph-sac. This startling announcement is evidently based on extended and careful 
observations, and it will make it necessary to re-examine the whole ground. 
