604 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
With regard to the affinities of Limulus the author proposes this 
genealogical tree — 
Arachnida Merostomata Crustacea 
In the origin of the mesoblast, separation of the nerve-cord from the 
epiblast, formation of the heart, number and position of eyes, grooves in 
the brain, &c., Limulus approaches the Chordata mere closely than it 
was thought to do, and the Arthropoda are, therefore, more nearly 
related to the Chordata than to the Vermes. 
e. Crustacea. 
Median Eye of Crustaceans.* — Prof. C. Claus discovered some 
years ago, in regard to the frontal eye of Cypris, that the nerve approached 
the visual cells from the outer side and that their ends were directed to 
the pigmented body ; in short that the eye was an inverted cup-eye. He 
has extended this observation with reference to other Ostracoda, Bran- 
chiopoda, Cladocera, Argulidae, Copepoda, Cirripedia, and Malacostraca. 
His general interpretation may be stated as follows in free translation. 
The three optic cups which compose the median eye of Crustaceans, 
and are perhaps phylogenetically related to the eye-spots on the apical 
plate of Annelid larvae, are, in contrast to what occurs in Insects, early 
removed from their ectodermic position, and are, like the brain, 
separated from the hypodermis and more or less dragged down. If we 
suppose that the three optic cups had originally a relation to one another 
and to the hypodermis like that which is observed in the three frontal 
eyes of Insects, not that a homology is asserted, we have to imagine that 
with the descent from the surface there was associated a convergent 
twisting towards one point. Thus we may explain the approximation 
of their convex surfaces, and the entrance of the nerves into the retina 
from the outer side. The pigment-forming cells, which lie peripherally 
to the stemma, and, comparable to an iris, around the mouth of the 
optic cup, would in the twisting or half-inversion be dragged down 
farthest, and would combine to form the two halves of each pigment 
sheath. On the other hand, the originally downward directed en- 
* Arb. Zook Inst. Univ. Wien (Claus), ix. (1891) pp. 225-66 (4 pis.). 
