Animal as a Whole. 71 



and there is nothing in it to renew the perplexity caused 

 by the presence of the " body " of the vertebra : the 

 nervous and vascular layers, are cup- like layers, open in 

 front, and tending to open at the bottom also, if the 

 point called the foramen of Sommering may be taken as 

 indicating such a tendency : the quasi-muscular layer is 

 also a ring, undivided in that part which forms the iris, 

 divided like the osseous ring into segments at the ciliary 

 processes by a series of cross-cuts : and so even, though 

 not quite so obviously, in the true muscular layer, external 

 to the eye-ball, for here it may be supposed that the 

 four recti muscles have been formed by the segmentation 

 of a primary ring like that of the iris or ciliary-processes. 

 As in the osseous layer there is nothing to represent the 

 " body " of the vertebra, so in the other layers, vascular, 

 nervous, muscular, and the rest, there is nothing to repre- 

 sent heart, or brain, or the like, except it be that the 

 muscular masses outside the eye are indications of 

 such centralization. The case, indeed, is one in 

 which the two great varieties of zone, the vertebral 

 and the annellar, are brought so closely together as to 

 make it easy to see how the one may pass into the 

 other. In the region of the tunica albuginea, the osseous 

 layer is practically external, for the parts which repre- 

 sent the muscular layer the ciliary processes and the 

 iris are within the eyeball. The case is substantially 

 that which is typical of the annellus. In the region 

 behind the tunica albuginea, on the other hand, the 

 relative positions of the osseous and muscular layers of 

 the coats of the eyeball are reversed, the latter, now 

 forming the straight and oblique muscles, being not 

 within, as in the last case, but without. The case, that 

 is to say, is one which is as typical of the bodiless ver- 

 tebra of which so much has been said, as the other was 



