278 



ANNELIDA. 



the animal. The circum-oesophageal ring (nri) is strongly marked, 

 communicating on each side with the ventral series of ganglia (o, p} 

 that extends throughout the entire length of the body, giving off nerves 

 to supply the different segments. Communicating with the posterior 

 aspect of the encephalic ganglia are several small ganglionic masses (i, fc, 

 I, m), which are joined together by delicate filaments, and apparently 

 represent the sympathetic system, inasmuch as from them are derived 

 filaments supplying the alimentary canal and the principal viscera. 



(713.) In Torrea 



vitrea (an Annelid the Fig. 138. 



transparency of which is 

 such that, when plunged 

 into sea-water, its pre- 

 sence is only distinguish- 

 able from the bright red 

 colour of its eyes and a 

 double line of violet- 

 coloured spots that ex- 

 tend along its back), 

 M. de Quatrefages* was 

 enabled to examine the 

 structure of the organs 

 of vision in a very satis- 

 factory manner. The 

 eyes in this species are 

 only two in number; and, 

 indeed, they constitute 

 by far the larger part 

 of the creature's head, 

 forming two very con- 

 siderable prominences 

 that are almost con- 

 joined in the mesial line 

 of the body. The in- 

 tegument, which is here 

 extremely thin and per- 



Structure of the eye in Torrea vitrea, and of the supposed 

 auditory apparatus in Arenicola (after Quatrefages). 1. 

 a a, integument passing in front of the eye, and forming a 

 transparent cornea ; b, c, granular cellular tissue enclosing 

 the globe of the eye ; d, external surface of reticular pig- 

 mental membrane ; f, internal surface of the same, seen 

 through the pupillary aperture ; e, the iris ; g, the crystal- 

 line lens ; g', optic nerve ; h, sheath of ditto, derived from 

 the dura mater ; i, k, vascular trunks forming a circle around 

 the base of the eyeball. 2. Auditory apparatus of an 

 Arenicola : a, acoustic nerve ; b, c, cellular tissue investing 

 the auditory capsule ; d, otolithic masses. 3. Auditory ap- 

 paratus of AmpMcoryne :. a, cellular tissue; b, auditory 

 capsule; c, otolith. 



fectly diaphanous, passes 



over the ocular globe, and evidently in this case performs the functions 

 of a transparent cornea (fig. 138, 1, a). A thick fibrous stratum, repre- 

 senting the sclerotic (d), encloses the eye, and becomes continuous with 

 the sheath, likewise fibrous (h), of the optic nerve (g'). The colourless 

 sclerotic presents upon one side a large irregularly-rounded aperture 

 that is partly closed by a sort of choroid of a brownish colour (6), in the 

 centre of which is an almost circular pupil surrounded with a border of 

 * Ann. des Sci. Nat. 1850. 



