98 THE PINEAL ORGAN 



free-swimming larvae, however, in the trochophore stage are in some 

 species characterized by the presence of one or two eye-spots, placed near 

 the apical plate, as in the trochophore larvae of some of the echinoderms 

 and many of the arthropods and molluscs. 



Taking Nereis, a marine worm, as an example of the class Polychaeta, 

 or worms with many bristles, we find a distinct head bearing on its dorsal 

 aspect two pairs of eyes. Each of these is connected with the superior 

 oesophageal ganglion or brain by a separate optic nerve (Fig. 62). The eyes 

 lie near the median plane, and consist of a cup-shaped vesicle closed 

 superficially by a smooth cornea (Fig. 63). This is formed superficially 

 by a non-cellular layer — the cuticle — which is continuous circumferentially 

 with the surrounding cuticle ; beneath this is a layer of flattened 

 hypoderm cells. The optic cup is filled with a viscous, non-cellular 

 material — the vitreous. This is divided by a constriction at the mouth 

 of the cup into a superficial lens-like structure, lying outside the cup, 

 immediately beneath the cornea, and the true vitreous which occupies 

 the cavity. The wall of the cup is formed by tall columnar cells, the 

 inner ends of which are clear and rod-like. These converge towards the 

 centre of the vitreous. The middle zone of the wall of the cup is deeply 

 pigmented, while externally are seen the nuclei of the cells and their 

 tapering outer ends, which are continued into a nerve-fibre layer and the 

 optic nerve. The mouth of the cup is constricted so as to form a pupil 

 of the immovable type, there being no iris or pupillary muscles. No 

 special ocular muscles are developed around the eyes and there would 

 thus be no independent movement of the eyes, the separate fields of vision 

 would overlap and the brain would receive impulses from a combination 

 of two binocular fields of vision. 



The larva of Nereis passes through a trochophore stage in which a 

 typical " apical plate " is formed, beneath which is the rudiment of the 

 cerebral ganglion. In the dome-like, upper segment of the trochophore 

 near the apical plate a pair of pigment spots, the larval eyes, are developed. 

 These are succeeded at a later stage by the appearance of another pair. 



An interesting feature in the further development of N. dumerilii is its 

 conversion by metamorphosis into a second form called Hetero-nereis, in 

 which one of the principal changes is a great increase in the size of the 

 eyes. This is associated with a change in the habits of the animal, from 

 a slow, creeping life at the bottom of the sea to one in which it swims 

 actively through the water. 



In a recent communication by R. S. Brown to the R. Soc. Edin. on the 

 anatomy of the Polychaete Ophelia cluthensis, he describes the eyes in this 

 species as being three in number (Fig. 64) : two anterior, right and left, 

 and one posterior on the left side, the right member of the posterior pair 



