MUSEUM OF COMPARATIVE ZOOLOGY. 195 



commissures ; but with a greater concentration of the nervous ganglia 

 toward one another, the sac is forced to occupy a position below the 

 pedal ganglia and their commissures. The relations of the different 

 ganglia to each other is even more definite than before, and can be more 

 readily understood from transverse sections than from sagittal ones. 

 The peripheral nerves from the cerebral, pedal, visceral, and abdominal 

 ganglia are well developed ; the principal changes from this time until 

 hatching are histological. 



The cerebral invaginations have become narrow and shorter, but are 

 still open to the exterior (Plate X. Fig. 124, iv.). The deeper portion 

 of the invagination, that in contact with the proliferated portion of 

 the cerebral ganglion, has become a solid and rounded mass (Plate X. 

 Fig. 122, lob. lat.), which is intimately connected with the ganglion 

 by means of fibrous outgrowths from its ganglionic cells. It is com- 

 posed of small deeply stained cells, which have undergone no such 

 histological change as those which compose the proliferated portion of 

 the brain. It forms a lobe on the antero-lateral face of each cerebral 

 ganglion (Plate X. Figs. 122, 124, 127). From this time forward the 

 principal change in the cerebral sacs consists in the gradual obliteration 

 of the lumen of the invagination. This is usually completed somewhat 

 later in the embryonic life ; but, as previously stated, the sacs have in 

 one instance at least been found open several days after hatching. Be- 

 sides this, there is no other connection now remaining between the 

 ectoderm and any of the ganglia, except such as is effected by means 

 of the peripheral nerves. 



The median proliferated portions of the cerebral ganglia now extend 

 dorsally farther than in the last stage, and their commissure is much 

 shorter (Plate VIII. Fig. 105, and Fig. F). 



The pedal ganglia (Plate VIII. Figs. 103 a , 109-113) have moved 

 forward, and are broadly in contact with the pleural ganglia. They 

 have become more compact, and rather more triangular in shape, than 

 before. From the ventral portion of each ganglion emerge four or 

 five large nerves, which terminate in the ventral wall of the foot ; from 

 the dorso-lateral region two nerves are given off to the lateral walls, and 

 the antero-ventral part of each ganglion tapers off into a stout nerve 

 running forward to the anterior wall. The connectives with the cerebral 

 ganglia are well developed (Plate VIII. Figs. 103, 107). 



The pleural ganglia (Plate VIII. Figs. 103% 111-113) are nearer to 

 the median plane than previously. The ventral posterior face of each is 

 closely joined to the corresponding pedal ganglion (Figs. 103 a , 112), 



