REPORT ON THE CRUSTACEA MACRURA. XIX 



development is undergone within the ovum until the embryo has reached the Phyllosoma 

 or Megalopa stage, it exists in the newly hatched animal. Whenever the two kinds are 

 found together, the oculus is the first formed, and therefore, according to Professor 

 Hartog, 1 who has given considerable attention to the development and structure of this 

 organ, it must be regarded as the primitive eye of the Crustacea. 



By investigating the anatomy of Cyclops and Diaptomus by the method of sections, 

 he has ascertained that this organ is much more complicated in structure than was 

 previously supposed. He says that Claus has demonstrated that it is formed in all cases 

 of a central pigmented mass, in which are half immersed three lenticular bodies or 

 crystalline spheres — two lateral and one central. 



The pigmented mass is structureless ; the colouring granules are situated at the 

 surface contiguous to the crystalline spheres. Each sphere is composed of radiating 

 elements or optical bacilli, the inner ends of which are applied against the pigmented 

 mass, while the peripheral segments contain a nucleus. 



He describes the oculus as being situated upon the terminal process of the brain, from 

 which the optic nerves originate, one for each sphere ; the nerve, instead of penetrating 

 into the pigmented mass, surrounds the outer surface of the crystalline sphere and 

 penetrates directly not far from its posterior margin. 



Claus has figured an analogous structure in the unpaired eye in the Phyllopoda 2 but 

 has not indicated its true significance. 



Dr. Hartog concludes that the unpaired eye, in all Crustacea that possess it, is com- 

 posed of three simple eyes placed anteriorly to the brain, with reversed optical bacilli, 

 receiving conductive fibres of the optic nerve upon their outer margin, and brought so 

 close together that these pigmented or choroid layers are combined into a single mass. 



Dr. Hartog further says that the eye which most nearly approaches the unpaired eye 

 in Crustacea seems to be that of the Planaria, and that according to Justus Carriere," 

 the structure of the two paired eyes in the Planaria is similar to that described by 

 Dr. Hartog in the simple eyes united in the middle line of Crustacea. It is therefore, he 

 says, more rational to refer the eyes of the Crustacea to such a primitive and ancestral 

 group as the Turbellaria, than to seek direct approximation between higher groups. 



It appears, therefore, that when the central eye is present in the embryo of the 

 higher Macrura, as may be seen in that of Crangon, Astacus, Palsemon, and the 

 Phyllosoma of the Palinurkke (where it only exists as a deciduous organ, and disappears 

 before the animal attains maturity), in most cases it is only represented by a mass of 

 pigment and that the crystalline spheres are seldom developed. In PL XIIa. figs. 2, 4, 

 a single sphere is shown in a specimen which was taken off Samboangan, in the Philippine 



1 De l'oeil impair des Crustaces, Comptes rendus, t. xciv. pp. 1130-1432, 1882. 



2 Clans, Zur Kenntniss des Baues und der Entwickelung von Branchipus stagnalis und Apus cancriformis, AbhandL 

 h. Gesellsch. Wiss. Gottingen, 1873. 



3 Archivf. mikrosk. Anat., Bd. xx. p. 160. 



