COMrOUND h'VES OF A nTlinoPonS. 325 



refractive body in the c-eiitro of eacii pit or tlio onnnatiilium, 

 wliicii has been described by several as tlio "crystalline rotic" in 

 tlie starlisli eye. The wliolc surface of tlie eye-bulb is covered by a 

 thin cuticle which is secreted by the cells wliose distal extremi- 

 ties lie on the general level of the external surface of the optic 

 bulb. In short, this external cuticular covering of the eye in a 

 starfish corresponds to the cornea, and the cuticular secretions at 

 the distal extremities of the retinal cells forming the walls of the 

 pit, correspond to the crystalline cone and the rhahdom of the 

 compound eye of an Arthropod. The colorless fluid substance 

 contained in the axial space of the invagination in the starfish 

 may be compared to the similar substance in the crystalline cone 

 of the ommatidium of SeroUs. The number of ommatidia in the 

 starfish increases at the periphery of the optic tract as in an 

 Arthropod. The spaces between the adjacent oinmatidial invagi- 

 nations are occupied by the supporting i>pithclial cells. The 

 optic nerve fibres pass into the deepest stratum of the ci>ithclial 

 layer, and running horizontally, join the main branch of the 

 ventral nerve cord of the ray. 



On the whole, the entire morphological arrangement of the 

 parts in the compound eye of an Echinoderm is strikingly 

 similar to that of an Arthropod, and the formation of the visual 

 organ in the Echinoderms, at least in one of the large sub-groups, 

 the Asferidce, by a series of ectodermic invaginations, assumes a 

 wider significance when we take into consideration the fact that 

 the compound eye of the Arthropods has a similar mode of 

 origin, although it is an independent adaptation. 



In the anatomical consideration of the Echinoderm eye I have 

 made no allusion to the so-called " eye-spot" on the ocular plate 

 of the sea-urchin. In regard to the function of this structure we 

 are by no means certain that it is visual in nature as it is usually 

 assumed. Valentin failed to discover any trace of dioptric appa- 

 ratus in it. More recently Fredericq* could not find any 

 evidence, anatomically and experimentally, that the structure in 

 question is an eye. '^La tache de pigment qxi'on y decritest une 

 pure fiction.'''' 



'Fredericq, Leon: Contributions Cl Vt'tude de-i Echinides. Archives des 

 Zoologie E.vperi. et Gener., Tome 0, 1876, p. 434. 



