504 SUMMARY OF CURRENT RESEARCHES RELATING TO 



The syn-cerebrum of the Tetradecapoda, Amphipoda, and Isopoda, 

 judging by Leydig's figures and his own observations on that of 

 Idotea and Lerolis, is built on a different plan from that of the Deca- 

 poda. The syn-cerebrum of the Phyllocarida is somewhat like that 

 of the Cladocera and Copepoda (Calanidae) ; being essentially different 

 from that of the majority of the Malacostracous Crustacea. The 

 Copepodous brain is an unstable, variable organ, but on the whole 

 belongs to a different category from the syn-cerebrum of other 

 Neocarida. 



We have then, probably two types of archi-cerebra, and three 

 types of syn-cerebra among existing Crustacea. 



Unpaired Eye of Crustacea.* — In most Crustacea, besides the 

 two compound eyes (fused together in the Cladocera), there exists 

 an unpaired median eye. It exists alone in most of the Copepoda, 

 and in all naupliiform larvae. Wherever the two kinds coexist in the 

 adult but not in the newly hatched larva, the unpaired eye is the first 

 formed, and must therefore be regarded as the primitive eye of the 

 Crustacea. By thin sections of Cyclops and Diaptomus, Mr. M. M. 

 Hartog has ascertained that this organ is of a much more complicated 

 composition than had been supposed. The pigmented mass is, so to 

 speak, structureless ; the colouring-granules in it are placed at the 

 surface contiguous to the " crystalline spheres." Each sphere is com- 

 posed of radiating elements, the inner ends of which are applied 

 against the pigmented mass, while the peripheral segments contain a 

 nucleus. The eye is 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, shirts the outer surface 

 of the crystalline sphere, and penetrates it directly not far from its 

 hinder margin. The author has also found in the Phyllopoda a 

 perfect analogy of structure with that just described in the Copepoda, 

 and therefore concludes that the unpaired eye in all the Crustacea 

 that possess it, is composed of three simple eyes, placed anterior to 

 the brain, with reversed optical bacilli, receiving conductive fibres of the 

 optic nerve upon their outer margin, and brought so close together that 

 their pigmented or choroid layers are combined in a single mass. 



A nearly identical structure may be detected in the Chaetognatha, 

 which have the triple eye of the Crustacea ; but, instead of being 

 median and unpaired, it is repeated on the two sides of the head ; 

 certain Planarians, Dendrocoelum lacteum for example, have two paired 

 eyes, which, according to Carriere, have the structure adopted by 

 the author for one of the simple eyes united in the median eye of the 

 Crustacea. 



It is probable that the eye of the Chaetognatha and Crustacea is 

 to be referred back to the type of the Planarians, but that the two 

 former groups have no direct relationship between them. 



Blood of the Crustacea, j — G. Pouchet is reported to find that the 

 differences seen in the blood of these animals is not, as Wharton 



* Comptes Eendus, xciv. (1882) pp. 1430-2. 



t Journ. Anat. et Physiol. (Robin) xviii. (1882) pp. 202-4. 



