230 



COMPARATIVE ANATOMY 



CHAP. 



in the Aldopidce reach a remarkable size ; many Sedentaria are eye- 

 less ; where, however, eyes occur they lie as small so-called eye-spots, 

 generally in great numbers, in the prostomium, at the part where 

 the brain is connected with the hypodermis. In Fabrida eye-spots 

 occur at the posterior end of the body; in some species of Sabella 

 on the tentacle gills. For the so-called lateral eyes of Polyoplithalmus 

 see below. The Echiuridce-, which live in mud or lurk in holes, 

 and the parasitic Myzostomidce, are blind. The absence of eyes 

 is characteristic of the whole class of the Prosopygia. The Piotatwia 

 possess an unpaired or a paired eye- spot lying on the brain, but this is 

 wanting or rudimentary in the adult condition in stationary forms. 

 DinopJiilus has 2 eye- spots in the prostomium. In the Chcetognatha 

 2 eyes lie on the dorsal side of the head, behind the brain, in the body 

 epithelium. 



Structure of the eyes. A comparative morphology of the eyes 

 of worms is at the present time a desideratum. At any rate it now 

 appears certain that the eyes in the various divisions need not be 

 homologous. The eyes which appear in pairs are perhaps homologous, 

 such as those developed in larvae of the Trochophom type in close 

 connection with the cerebral rudiment; these very often disappear 

 through metamorphosis, or degenerate. As a detailed account of 

 these always more or less complicated organs is here impossible, we 

 select a few for brief description, viz. the eyes in Capitella, in Aldope 

 (the most highly developed eye among worms), in a Chcetognathan, and 

 in Hirudo. 



1. The eye of Capitella. The numerous eyes (ocelli) of Capitella lie 

 in the prostomium at the part where the brain fuses with the hypo- 

 dermis. The following are the 

 elements of a single eye. We 

 find slipper -shaped refracting 

 cells, whose outer portion is 

 homogeneous and transparent, 

 while the inner part contains 

 pigment. Each of these cells 

 is continued as a nerve fibre 

 towards the brain, this nerve 

 fibre entering a ganglion cell of 

 the central optic lobe. The 

 refracting cells of the eyes are 

 connected together by thread 

 cells of the hypodermis. The 

 cuticle is arched over the 

 eye. 



2. The eye of Aldope (Fig. 

 153). The two eyes of Aldope 

 stand out spherically, one on 

 They are covered by a thin layer of hypo- 



fl 



FIG. 153. Section through the eye of an Alciope 

 (Callizona Grubei). hy, Hypodermis ; c, cornea ; I, 

 lens ; fl, eye fluid ; p, pigment of the retina ; r, retinal 

 cells ; st, rods ; go, ganglion opticum of the brain 

 (after Carriere). 



each side of the head. 



