Prof. E. Claparede 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 di^ follicular^ -, and he op- 

 poses it to that of the Annelida, according to his own researches 

 on the Oligochfeta and those of M. de Quatrefages on the Poly- 

 chseta. This distinction cannot be made so absolute. Certain 

 Annelida Polychseta have a follicular nervous system as well as the 

 Hirudinea. This is the case, for example, in Nereilepas caudata 

 &c., as I 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 Oligochseta, is restricted in the Polychseta to a 

 small number of families {Capitellea, Ariciea, Spiodea, Syllidea, 

 Eunicea), and apparently even only to certain i-epresentatives of 

 these families. 



The terminations of the nerves in the Annelida have hitherto 

 been studied only by myself, M. Keferstein, and M. KoUiker. 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 f is in reality still very imperfectly known, 

 even in Alciope, notwithstanding the investigations of M. Ley- 

 dig. In connexion with this, I cannot abstain from mention- 

 ing an opinion of J. Miiller's, which has fallen into oblivion. 

 We owe to that great physiologist J 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. 



t When M. Victor Cariis (Handbuch der Zoologie, p. 430) ascribes 

 auditory ca])sules to the majority of the Annelida, he deceives himself very 

 greatly. The existence of these organs is peculiar to a very restricted 

 number of species. 



X " Mcmoire sur la structure des yeux chez les Mollusqucs Gastcropodes 

 et quelqucs Anuelides," Ann. Sci. Nat. tome xxii. 1831, p. 23. 



