424 INTRODUCTION TO CRANIATA [CH. 



which breaks loose from the skin and lies in the mouth of the 

 secondary optic vesicle (Fig. 211). The skin outside the lens forms 

 the cornea, which is transparent. The cornea is joined to the edges 

 of the sclerotic capsule and thus completes the boundary of the eye- 

 ball, as the fully-elaborated sense-organ may be termed. If the above 

 description has been followed it will be seen that in a Craniate 

 light must reach the visual cells through their basal and not 

 through their visual ends. As this is contrary, to the almost 

 universal rule obtaining throughout the animal kingdom, we cannot 

 believe it to be a primitive arrangement. Rather we must believe 

 that when the eye was being evolved the rods of the visual cells were 

 directed towards the light, and that the epithelium of which they 

 form a part was exposed and not rolled up into a neural tube ; in a 

 word, that the front portion of the nervous system of Vertebrate at 

 any rate was once a plate of sensitive skin. It is most suggestive 

 to note that in the larva of the Hemichorda we find such a plate 

 with two eye-spots at the apex of the prae-oral lobe. 



The external layer of the skin or ectoderm of Craniata is quite 

 peculiar in the animal kingdom, in that it consists not of one, but of 

 many layers of cells. On closer inspection, however, it is seen that 

 the deepest layer, consisting of columnar cells alone, really repre- 

 sents the ectoderm of the other phyla. This layer 

 instead of becoming directly converted into cuticular 

 substance externally, as, for example, in the Arthropoda, buds off 

 layers of cells from its outer surface which, as they become pushed 

 further and further out, become bodily converted into horny mattei 

 and scale off. The ectoderm rests on a specially firm bed of con- 

 nective tissue called the derm is. 



A very peculiar feature in the Craniata is the character of the 

 Nerves scattered sense -cells of the skin. .These end in 

 sense filaments embedded in the ectoderm, not pro- 

 jecting beyond it. These filaments have however grown enormously, 

 and with their growth the bodies of the cells with the nuclei have 

 come to lie deep down in the body and so have become quite in- 

 distinguishable from ordinary neurons. Here they form metamerically 

 arranged packets of cells lying at the side of the nerve-cord and 

 laiown as the spinal ganglia. They are connected with the nerve- 

 cord by their basal outgrowths or nerve-tails, which constitute the 

 dorsal roots of the spinal nerves corresponding to the dorsal sensory 

 nerves of A mphioxus. To each myotome a motor nerve is given off, 

 as in Amphioxus, but in the Craniates the fibres of this nerve are 



