AND EMBRYOLOGY OF LIMULUS. 27 



This layer of cells is succeeded by a thin, slightly curvilinear, transverse strip of 

 connective tissue, passing through the entire eye, and behind it are the loose, nucleated 

 spherical cells forruing the ganglion opticum, among which the fibres of the optic nerve 

 pass. 



The brain of Cermatia forceps, as shown by several sections, is modelled on the same 

 plan as we have observed in Bothropolys and so far as we see, the myriopod brain corre- 

 sponds more closely in its general form and structure with that of the Insects than of the 

 Crustacea. The large, thick optic nerve arises from the upper side of each hemisphere. 

 The median furrow above is deep, and on each side is a mass of small ganglion-cells ; also a 

 mass in the deep sinus below the origin of the optic nerve, and another mass on the inferior 

 lobe extending down each side of the oesophagus, probably near or at the origin of the 

 posterior commissure. These masses, i. e., those on the upper and under side of the brain, 

 connect on each side of the median line, and in this respect the brain is as in 

 Bothropolys. There are no large ganglion cells, as in the Crustacea and in Limulus. 



It will be seen from this brief account that the eyes of Limulus differ from those of 

 Myriopods in wanting the lens-epithelium, the rods, and a ganglion opticum, which are 

 present in other Arthropods, both tracheate and branchiate. 



The Brain and its Internal Structure. 



Several years ago, before the present interest in the study of the brain of Arthropoda 

 had arisen, I made an attempt to study the brain or supi'a-oesophageal ganglion of the 

 horse-shoe or king crab. Mr. T. D. Biscoe kindly cut a number of sections for me. 

 These were unstained, and owing to interruptions were not examined until the past 

 winter, when with the aid of a large number of other sections made by Mr. N. N. Mason of 

 Providence, R. I., I have been enabled to present the following results. The brain was 

 in some cases stained with osmic acid in the manner described by Dietl ^ and adopted by 

 Newton — being taken from the living animal and allowed to remain in the osmic acid 

 from twenty to forty hours. 



The best results, however, were obtained from sections of two brains, one of which had 

 been several years in alcohol, and which took the osmic acid stains very evenly ; better 

 results ensued by staining, after hardening the brain for two or three days in alcohol ; 

 the brain is so large that it does not harden readily, when put fresh and living in osmic 

 acid alone. 



When sections were not properly stained in the centre with osmic acid, they were further 

 treated with a picro-carmine stain with good results. Mr. Mason embedded the brain, when 



' The following articles bearing on the brain of the Ar- Flogel. Ueber den einheitlichen Bau des Gehirns in den 



thropods have been consulted ; the actual bibliography of verschiedenen Insectenordniingen. Zeitschr. wissensch. 



'he subject being somewhat fuller. Zoologie, Bd. xxx, Suppl. 1878, p. 556. 



Ofsiannikoff. Ueber die feinerer Structur des Kopfgang- Neivlon. On the brain of the Cockroach, Blatta orientalis. 



lions bei den Krebsen, besonders beini Palinurus locusta. By E. T. Newton. Quart. Journ. Microscopical Science, 



Von Ph. Ofsiannikof Mem. Acad. Imp. So. St. Peters- July, 1879, p. 340. 



bourg. Tom. VI. No. 10, 1863. Krieger. Ueber das Centralnervensystem des Fluss- 



Dietl. Die Organization des Arthropodengehirns. Von krebses. Von K. R. Krieger. Zeitschrift fiir wissenschaft. 



M.J. Dietl. Zeitschr. wissensch. Zool. Bd. 27, 1876, p. 488. Zoologie. Bd. xxxiii, Jan. 23, 1880, p. 527. 



