262 RECORD OF CURRENT RESEARCHES RELATING TO 



vary in size and in external characters, is always formed on a common 

 type ; it generally has a spherical form and consists of two principal 

 portions — an outer, which is nothing more than a modification of the 

 general cuticular investment, and which forms the dioptric organ, and 

 an inner part, which is directly connected with the nervous system 

 and forms the perceptive organ or retina. There is no proper 

 sclerotic, but the retina is invested by a thin cuticle which is a con- 

 tinuation of the cerebral capsule. The first or tegumentary portion 

 may be cuticular or hypodermal : the former is relatively highly 

 homogeneous and pellucid ; not only is there no cuticular cornea 

 developed but in Nereis the cornea is even thinner than the surround- 

 ing integument. The hypodermal portion, which the author proposes 

 to call the dioptric intermediate body, is distinguished by its great 

 transparency and its considerable power of refracting light ; further 

 than this we can make out two different forms, inasmuch as the body 

 may be (as in Nereis, Aphroditis, Polynoe, &c.), a portion of the hypo- 

 dermis which consists of radially arranged tubular cells which 

 increase in length from the periphery towards the axis, and have their 

 nuclei placed at the basal or retinal end ; in the others the central 

 portion of the body is occupied by a homogeneous cuticular structure 

 which appears to form the lens proper ; the cells set around it and 

 investing it are the corneo-epithelial cells, while the lens itself is 

 either plano-corneal (Eunice) or convex-corneal (Alciope). It con- 

 sists of a homogeneous mass which appears to be gelatinous rather 

 than chitinous in character. In Alciope the lens consists of three 

 concentric layers, of which the outermost is granular and appears 

 to be the softest ; on the other hand, in Nephthjs the granular 

 matter occupies the central portion. 



The retina has the form of a goblet hollowed out in the middle ; 

 merely the terminal outspreading of the optic nerve, it consists 

 of two parts continuous one with the other ; internally there is a 

 layer, generally very thin, of optic fibres, and an outer, thicker, pali- 

 sade layer. The elements of this last are tubes which are generally 

 prismatic in form, and are continuous internally with the optic fibres, 

 while externally they are attached to the limiting membrane. That 

 the constituent bodies are not histologically elementary organs is 

 shown by the fact that they contain two nuclei at least ; in some, e. g. 

 Eunice and Alciope, there is, further, a median retinal nucleus, and 

 the wall of the tube between this and the outer nucleus is consider- 

 ably thickened, is highly refractive, and is composed of alternating 

 lamellae with different refractive indices ; this portion, which may 

 well be known by Greef's name of " tube-rod," always contains a 

 granular pigment which is black when present in considerable 

 quantity. 



When we compare the parts here described with the arrangements 

 that are found among the Tracheata we see that we have to do with 

 homotypical structi.res ; the only difierence, and this is not constant, 

 is that in the Tracheata the rod forms an axial, while in the Chaeto- 

 poda it is a j)arietal portion of the retinal tube. At the same time it 

 is to be noted that, notwithstanding this homotypy, the eyes of these 



