6 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



glion cells, froiii which fibers arise after passing through the mj-eloid substance ; there becoming 

 broken up into a tangled mass of fibrilla', which unite finally to form the fibers constituting 

 the nerves of the appendages. Without doubt also a few commissural fibers from the procere- 

 bral lobes pass into each post-cerebral ganglion so as to afford the means to the cerebral lobes 

 (l)rimi inter pares, as happily styled by Leydig) of coordinating the nervous power of the other ganglia, 

 their histological and morphological equivalents. It should be said that although Leydig's view as 

 to the relations of the nerve-fibers to the myeloid substance may be the correct one, yet though it 

 may apply to the Annelids, it may not be so general an occurrence in the Arthropods. It seems to 

 us, though we are still o[)en to conviction, that the transverse and longitudinal commissural fibers, 

 which undoubtedly arise from the cortical ganglion cells, have little or nothing to do with the 

 myeloid substance. This latter substance does not exist in the nervous system of the vertebrates, 

 and just what its nature and function clearly are in the invertebrates has yet to be worked out. In 

 the hands of a skillful and expert histologist, much light will yet be thrown upon this diflicult 

 subject ; certainly the present writer has not the qualifications for the task. His own opinion from 

 what little he has seen is, that the myeloid substance is the result of tlie splitting up into a tangled 

 mass of very fine flbrilhe of certain of the fibers thrown off from the monopolar ganglion cells, i.e.^ 

 such fibers as do not go to form the maiu longitudinal commissures. It should also be borne in 

 mind that iu the embryo the ganglia are comi)osed of ganglion cells alone, with few if any primitive 

 fibers. 



MOKPHOLOGY OP THE BRAIN. 



The brain of the Isopods and Amphipods is a synccrchnim, though far less complicated than 

 in the Decapoda. It will be remembered that Professor Lankester in his memoir on Apus desig- 

 nates the simple brain of that crustacean as an archiccrehrnm, while the composite brain of " all 

 Crustacea, excepting Apus, and possibly some other Phyllopods," he denominates a syncercbrum. 

 In our Monograph of N. A. Phyllopoda, if .iOS, we adopted the view that the brains of all Crustacea 

 except the Phyllopoda and Merostomata were syncerebra, aud we divided the syncerebrum into 

 three types; adding that the syncerebrum of sessile eyed Crustacea (Edriophthalma) was built on 

 a different plan from that of the Decapoda. 



Fig. 1 has been drawn to give a general view of the nervous centers of the head, including the 

 first thoracic segment aud its ganglion. It has been drawn with the camera from a number of 

 sections, especiallj' those represented by Figs. 5-S, so that it is believed to be approximately 

 correct and not merely a schematic plan. The section passes through the head on one side of the 

 oesophagus, which of course is not represented in the sketch ; being so near the median line it 

 does not involve the optic lobes aud eyes, which, especially the latter, are ou the extreme side of 

 the body, so that these organs could not well be shown in the drawing. The general relation of 

 the nervous system to the body walls, to the stomach and the appendages are made obvious in the 

 sketch, aud their description need not detain us. It should be borne iu mind that the mouth and 

 oesophagus open between the mandibles. They are shown in Fig. 5. The end of one of the 

 ovarian tubes is seen to overlie the pyloric end of the stomach ; it does not pass into the head. 

 The drawing of the heart is somewhat diagrammatic, as it was not well shown in the sections, but 

 its iJosition is believed to be approximately correct. The sympathetic nerve was not discovered. 



As seen in Fig. 1, the brain or supraiesophageal ganglion is a composite mass or group of 

 four pairs of ganglia, *. e., (1) the brain proper or procerebral lobes, (2) the optic ganglia, (3) the 

 first antennal, and {•!) the second antennal lobes. These lobes are quite separate from each other 

 iu the Isopoda aud Amphipoda as compared with the Decapoda. 



THE PROCEREBRUM OR PROCEREBRAL LOBES. 



These constitute the brain proper, and have been usually called the "cerebrum" or "cerebral 

 lobes." As, however, they are not the homologues of the lobes of that name in Vertebrates, either 

 structurally or functionally, we would suggest that the ganglion be termed the procerebrum and 

 the individual lobes the procervhral lohcs, not only iu allusion to its position iu advance of all the 



