24 MORPHOLOGICAL TYPES 



vitreous humour, the anterior chamber contains the aqueous 

 humour. 



There is an important point to notice in connexion with the 

 retina. In Amphioxus the cells which are sensitive to light 

 line the central canal of the nerve-tube ; in all higher forms, 

 that portion of the wall of the nerve-tube in which these cells 

 lie is bulged out sideways to form the eye. The sensitive cells 

 are, however, still morphologically on the inner side of the wall 

 of the brain (i.e. adjacent to the central cavity or the cavity of 

 the optic vesicle). The nerve-fibres which convey the im- 

 pulses away from the retina cannot go through this cavity, they 

 must run in the wall of the optic vesicle and of the brain. In 

 so doing the nerve-fibres must therefore pass between the 

 sensitive cells and the lens. This means that the image of 

 the seen object reaches these sensitive cells after passing 

 through the nerve-fibres. A retina of this kind is called in- 

 verted, and is characteristic of the paired eyes of all chordates. 

 The pineal eyes (described below in connexion with the brain) 

 have an erect retina, for here the nerve-fibres leave the retina 

 on the side away from the lens. With regard to the paired 

 eyes, it is essential to realise that the cavity of the primitive 

 optic vesicle is not the same as that of the definitive eyeball (or 

 posterior chamber). In the process of conversion of the optic 

 vesicle into the optic cup, the cavity of the vesicle has been 

 obliterated. The eyes are not immovable, but can be turned 

 in various directions. This is effected by the myotomes of 

 the first three segments, which are modified into so-called 

 eye-muscles. The description given above applies to the 

 paired eyes of all chordates ; the eyes of the lamprey are, 

 however, somewhat degenerate. 



Petromyzon has so-called auditory organs or ears, but it 

 must be remembered that these organs primitively do not serve 

 for the purpose of hearing, but are organs of balance. They 

 take the form of sacs on each side of the brain behind the eyes, 

 giving off canals in the form of half- hoops, each end of which 

 opens into the sac. These are the semicircular canals ; each 

 one bears a swelling or ampulla containing a statolith or organ 

 of balance (see p. 395). In all vertebrates above Petromyzon 



