64 THE ANATOMY OF IXVERTEBRATED ANIMALS. 



And as respects the ectodermal cells which constitute the 

 fundamental part of the organs of the special senses, it is 

 becoming clear that the more perfect the sensory apparatus, 

 the more completely do these sensigenous cells take on the 

 form of delicate rods or filaments. Whether we consider the 

 organs of the lateral ]ine in fishes and amphibia, the gusta- 

 tory bulbs, the olfactory cells, the auditory cells, or the 

 elements of the retina, this rule holds good. 



Every one of the organs of the higher senses makes its 

 appearance in the animal series as a part of the ectoderm, 

 the cells of which have undergone a slight modification. In 

 the case of the eye, accessory structures, consisting of vari- 

 ously-colored masses of pigment, which surround the visual 

 cells, and of a transparent refracting cuticular or cellular 

 structure which lies superficially to them — a rudimentary 

 choroid and cornea — are next added. The highest form of 

 compound Arthropod eye differs from this only in the differ- 

 entiation of the layer of sensigenous cells into the crystalline 

 cones and their appendages, and it has not been clearly made 

 out that the simple eyes of most other Invertebrata have 

 undergone any further change. 



But in Nautilus the nerve-cells and choroid line the walls 

 of a deep cup open externally ; which, though its development 

 has not been traced, may be safely assumed to result from 

 the involution of the retinal ectoderm. It may be compared 

 to an arthropod compound eye become concave instead of 

 convex. 



In the higher Cephalopoda, the margins of the ocular 

 pouch unite and give rise to a true cornea, which, however, 

 frequently remains perforated, and a crystalline lens is de- 

 veloped. In the higher Vertebrata the retina is still a modi- 

 fied portion of the ectoderm. For, inasmuch as the anterior 

 cerebral vesicle is formed by involution of the epiblast, and 

 the optic vesicle is a diverticulum of the anterior cerebral 

 vesicle, it necessarily follows that the outer wall of the optic 

 vesicle is really part of the ectoderm, its inner fa<re being, 

 morphologically, a portion of the surface of the body. The 

 rods and cones of the vertebrate eye, therefore, exactly corre- 

 spond with the crystalline cones, etc., of the Arthropod eye; 

 and the reversal of the ends which are turned toward the 

 light in the Vertebrata is a necessary result of the extraor- 

 dinary change of position which the retinal surface undergoes 

 in them. 



In the part of the ectoderm which takes on the auditory 



