Prof. B. Claparéde on the Structure of the Annelida. 357 
system, similar to that of the Hirudinea. I confess that I have 
been unable to discover it ; but I feel that this negative result is 
of no great weight in so difficult an investigation. I am, how- 
ever, astonished to find that so many other observers have had 
‘no better fortune than myself in perfectly similar endeavours. 
_ M. Leydig has described in the Hirudinea a structure of the 
nervous centres which he characterizes as follicular*; and he op- 
poses it to that of the Annelida, according to his own researches 
on the Oligochzta and those of M. de Quatrefages on the Poly- 
cheta. This distinction cannot be made so absolute. Certain 
Annelida Polychzeta have a follicular nervous system as well as the 
Hirudinea. This is the case, for example, in Nereilepas caudata 
_ &e., as [shall show hereafter. Others present nothing of the kind. 
__ The structure of the nervous system varies, however, astonish- 
ingly in the series of the Annelida; the distribution of the 
‘nerve-cells especially is subject to a multitude of modifications 
which we shall point out in particular cases. In the ventral 
chain, the cells belong chiefly to the ventral surface and the 
sides, as M. Leydig has already noticed. The existence of large 
tubular fibres on the dorsal surface of the nervous chain, so 
general in the Oligochzta, is restricted in the Polycheta to a 
small number of families (Capitellea, Ariciea, Spiodea, Syllidea, 
_ EBunicea), and apparently even only to certain representatives of 
families. 
_ The terminations of the nerves in the Annelida have hitherto 
been studied only by myself, M. Keferstein, and M. Kolliker. Nu- 
_ merous observations on this subject will be found in the present 
memoir. All these terminations seem to be in relation to the 
function of touch. The nervous expansion of the organs of 
sight and hearing ¢ is in reality still very imperfectly known, 
even in Alciope, notwithstanding the investigations of M. Ley- 
dig. In connexion with this, | cannot abstain from mention- 
an opinion of J. Miiller’s, which has fallen into oblivion. 
e owe to that great physiologist { an excellent figure of the 
central nervous system and of the eyes of the Nereides, a figure 
to which his successors have added nothing very positive. In 
his opinion, the organ which we now call the crystalline is not 
a dioptric medium ; he denies its transparency, and regards it as 
a terminal inflation of the optic nerve. Although the trans. 
* The observations of M, Baudelot upon Clepsine (Ann, Sci. Nat, 
tome iii. 1865, p. 126) are a complete confirmation of this. 
+ When M. Victor Carus (Handbuch der Zoologie, p. 430) ascribes 
auditory — to the majority of the Annelida, he deceives himself very 
=: X e existence of these Organs is peculiar to a very restricted 
number of species. 
t “ Mémoire sur la structure des yeux chez les Mollusques Gastéropodes 
et quelques Annélides,” Ann, Sei, Nat. tome xxii, 1831, p, 23, 
