STEUCTUEE OF THE BEAIN OP THE SESSILE EYED CEUSTACEA. 7 



other ganglia, but since it stands as the co-ordinating, regulating ganglion, the first in importance 

 of all the ganglia. 



As regards size, the procerebral lobes are more than double that of the other ganglia ; they 

 bulge out dorsallj^ and backward, so as to conceal from above the anteunal and mandibular 

 ganglia. Plate 1, Fig. 2, represents a section through the lobes in front of the commissure, 

 showing at «, h, the dorso-frontal group of ganglion cells, those nearest the myeloid substance 

 sending fibers downward (fb) to form a part of the oesophageal commissure. At Fig. 3, a section 

 farther back aud passing through the commissure, the fibers are seen to pass directly through the 

 myeloid substance along the inner side of the commissure. Fig. 4 represents a still more posterior 

 section ; this shows distinctly the origin of the fibers of the transverse commissure (tr. c) from the 

 ganglion cells of the upper aud outermost portion of the lobes. The commissure is seen to be 

 composed of three bundles of fibers — an upper, middle, and lower or ventral ; the space between 

 the upper and middle bundles being filled with myeloid substance. 



Vertical sections of the procerebral lobes are seen in Figs. 5 to 8. Fig. 5, which passes 

 through the median line of the head, through the mouth, oisophagus, and the median line of the 

 stomach, shows the procerebral lobe on one side of the commissure; and, below, the second 

 maxillary aud maxillipedal ganglia. Fig. 7, passing through one side of the first anteunal 

 ganglion, shows the procerebral lobe nearly separate from the anteunal lobe. Fig. 8 represents a 

 .section passing through the main commissure and a portion of the procerebral lobe. 



Horizontal sections from the top of the head downwai'ds are seen in Figs. 9 to IS. Fig. 9 

 represents a section through the upper part of the procerebral lobes ; Fig. 10, through the lobes 

 above the transverse commissure ; Fig. 11, through the entire procerebrum, near the origin of the 

 optic ganglia aud optic nerves. 



THE OPTIC GANGLIA AND OPTIC NERVES. 



The eyes being smaller iu Asellus than in most other genera of Isopods, particularly Oniscus 

 aud Porcellio, the forms figured and described by Leydig; the optic ganglion and nerve are also 

 much smaller, while the eyes being set farther back on the sides of the head, the ganglion aud nerve 

 are directed obliquely backward, so that a series of vertico-frontal sections pass through the brain 

 before reaching the optic nerve. PI. IV, Figs. 19-21, represent these organs. Fig. 19 shows the 

 procerebral lobes, and on the left the optic ganglion and the optic nerve leading to the eye. Fig. 

 20 represents a section just belaud the procerebral lobes, passing through the hinder edge of the 

 cortical layer of ganglion cells. Fig. 21 is an enlarged view of the sam e. The optic lobe is divided 

 into two parts, the inner connected with the procerebral lobe, with an abundant supply of gajUgliou 

 cells, while from the smaller, outer division arise the fibers which unite to form the optic nerve, 

 which divides at or just beyond the middle into several branches sent to the eyes. These branches 

 are seen to end in slightly bulbous expansions among the small retina cells, forming the deep- 

 brown pigment-mass in which the lenses are imbedded. 



The first antennal ganglia (Figs. 1, 7, and 12). — The relations from a side view to the other parts 

 of the brain are seen iu Figs. 1, 7, and la. It will be seen that the ganglion is much freer from 

 the procerebral lobes thau in the Becapoda. It may be seen from above, when looking down upon 

 the brain, projecting somewhat in advance of the procerebral lobes, the first antennal nerve arising 

 from the upper and anterior side, ascending a little at its origin, and passing liorizontally into the 

 base of the antenna. Fig. 12 represents a horizontal section through the lobes, showing the gan- 

 glion cells, the myeloid substance, and the origin of the antennal nerves. 



The second antennal lobes (Figs. 1, 7, la, U to 16). — The second antennal ganglion lies directly 

 beneath the upper or first anteunal lobes, aud appears to be slightly larger thau the latter, the 

 nerves being larger, corresponding to the much larger size of the second antenna. It will be seen 

 by reference to Figs. 14 to 16 that the oesophagus passes between the lower part of the lobes, which 

 are almost wholly separate. (Figs. 17 and 18, which represent sections just below that I'epresented 

 by Fig. 16, are introduced to show the oesoi)hageal commissures and their ganglion cells on each 

 side of the oesophagus.) 



