284. MARGARET ROBINSON. 
membrane with that over the eye-stalks. At the anterior apex 
of the triangle there is a dorsal elevation, which is also covered 
by chitin. This elevation, looked at from above, makes the 
triangle appear to have a knob for its anterior apex. 
On removing the chitin it is found to be lined by a thin 
layer of ectoderm. On removing the dorsal part of this ecto- 
derm and a small piece of the anterior portion of the brain, 
the eye can be seen lying in a blood-space just dorsal to the 
brain. It has the appearance of a black Y, which is slung 
on to the ectoderm by two slender threads which swell out in 
the concavities formed by the arms of the Y, and then narrow 
again as they approximate to each other. Surrounding the 
posterior ends of these threads is a semicircle of pigment 
corpuscles lying in the brain. 
Minute Anatomy.—lI have cut transverse and horizontal 
sections through the brain in Palemon serratus and 
Virbius varians, and have had at my disposal some made 
by Professor Weldon through that of Pandalus. The figures 
were made from transverse sections through the brain of 
Virbius (No. 3), being the most anterior section. In cutting 
the sections the chitin covering the ectoderm broke away, so 
that it is not shown in the drawings except in the posterior 
dorsal groove. . 
By means of sections it is seen that the ¥ consists of two 
large pigment cells, and that the supporting strings consist in 
their narrow anterior diverging parts of ectoderm, while their 
swollen parts in the concavities of the ¥ and the narrow stalk 
consist respectively of the thickened and narrow ends of 
club-shaped nerve-end cells. 
Ectoderm.—In the anterior sections the ectoderm lying 
dorsal to the median eye is thickened. I do not think that 
this thickening is due to obliquity in the sections, as it is present 
invariably, but rather that we are here cutting through the 
knob-like prominence mentioned above. Going further back 
an inward prolongation of the ectoderm is seen on either side. 
These prolongations are seen to converge ventrally as we trace 
them backwards. They join the thickened ends of the nerve- 
