SENSES OF THE ANNELIDANS. 279 



a deep blue colour. Through the pupillary opening it may be perceived 

 that the interior of the eye is lined by the choroid, and that the whole 

 interior of the ocular capsule is filled up with a vitreous humour so ab- 

 solutely transparent that the crystalline lens situated in its centre seems 

 to be in connexion with nothing. On the outside of the eye the optic 

 nerve can be plainly seen arriving at the eyeball and expanding to form 

 the retina. The eyes of other Annelidans are, however, when present, 

 by no means so easily examined; they may, however, from the re- 

 searches of Miiller*, "Wagner f, EathkeJ, and Siebold, be briefly stated 

 to consist of a round transparent medium or lens enclosed in a layer of 

 pigment, and provided posteriorly with a retinal expansion. 



(714.) An apparatus to which the functions of an organ of hearing 

 have been attributed by several eminent anatomists is met with in 

 some Annelidans. Grube and Stannius || first announced a very remark- 

 able structure in Arenicola, the existence of which has been confirmed 

 by subsequent observers, that certainly resembles very closely in its 

 conformation an organ common among the Mollusca, to which a similar 

 function has been generally conceded :. this consists of a transparent 

 membranous capsule (fig. 138, 2 & 3, a, >, c) enclosing a fluid, wherein 

 one, or sometimes several minute bodies, having every appearance of 

 otoliths, are suspended. M. de Quatrefages describes these auditory 

 capsules as being situated in the first or second segment of the body, one 

 on each side of the opening of the oesophagus, and observes that a nerve 

 of considerable size is distinctly traceable in them. 



(715.) Many of the smaller marine Annelids are luminous ; their 

 luminosity, however, is not a steady glow like that of the glow-worm 

 or fire-fly, but a series of vivid scintillations (strongly resembling those 

 produced by an electrical discharge through a tube spotted with tin-foil) 

 that pass along a considerable number of segments, lasting for an instant 

 only, but capable of being repeatedly excited by any irritation applied 

 to the body of the animal. These scintillations may be observed even in 

 separated segments if they be subjected to the irritation of a needle- 

 point or of gentle pressure ; and it has been ascertained by M. de Quatre- 

 fages that they are given out by the muscular fibres in the act of con- 

 traction^". 



* Ann. des Sci. Nat. t. xxii. t Icones Physiologicse, pi. 28. 



J De Bopyro et Nereide, pi. 2. figs. 4 & 5. 

 Lehrbuch der vergleichenden Anatomic, p. 200. 



I Lehrbuch der vergleichenden Anatomie von Siebold und Stannius, p. 201. 

 *[[ See his Memoirs on the Annelida of La Manche, in Ann. des Sci. Nat. ser. 2. 

 t. xix. and ser. 3. t. xiv. 



