Eyes of Molluscs and Arthropods. 625 



»retinal cells«. They are homologous witb the retinophorae of Pecien 

 and other Mollusca (the colorless cells of Gasteropod eyes); this is suffi- 

 ciently well showu by the presence of the axial nerve fibre, and the 

 double nature of the rods. It should also be indicated by the presence 

 of two nuclei, one of which Grenacher has probably overlooked. The 

 jiLimitanszellen« are ganglionic cells exactly similar to, and homo- 

 logous with. the ganglionic cells, especially those of the inner layer, of 

 the eye oiPecten. Consequently they represent modified pigment cells of 

 the hypodermis, and are homologous with the retinulae of the Gastero- 

 pod ommatidia. Their prolongations form the external, nervous net- 

 work of the rode. The limiting membrane corresponds with the corneal 

 layer of the cuticula, and probably protects loops of the axial fibre, 

 similar to those of Pecten. 



Chapter II. Crustacea and Insects. 



Penaeus. 



The great Impetus that modern Zoological science has re- 

 ceived from comparative anatomy, has not been due so much to more 

 subtle or able comparisous, as to a more perfect knowledge of the struc- 

 ture of single forms. It would not be too much to say that a perfect ac- 

 quaintance vsdth the anatomy of a single, typical Arthropod eye, with its 

 various stages of structural and functional development, would furnish 

 US vsäth a key to many of the most difficult problems concerning the 

 comparative anatomy and phylogenetic development of the visual 

 Organs, not only in Arthropods, but in other Invertebrates, as well as 

 Vertebrates. In proportion as our knowledge of individuai forms is 

 less perfect, or built upon false foundations, so will our deduction 

 from these false premisses not merely be wrong in the same proportion, 

 but exaggerated a thousand fold with every step, and finally entangle 

 US in such a labyrinth of false deductions, that future progress would 

 be well nigh impossible. We must expect a certain amount of struc- 

 tural uniformity in those organs which have to carry, by the same 

 means, the same forms of energy to similar perceptive centres, and that 

 the greatest uniformity should prevali in the most essential parts. My 

 studies upon the nerve endings in the Mollusca have induced me to 

 believe that a uniformity of structure prevails in the essential Clements 



Mittheilungeii a. d. Zoolog. Station zu Neapel. Bd. VI. 42 



