102 MEMOIRS OP THE NATIONAL ACADEMY OP SCIENCES. 



glioii cells, froui wliich fibers arise after passing tlirongli tlie myeloid substance; there becoming 

 broken ui> into a tangled mass of flbrillic, whicU unite tinally to form the tibers constituting 

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

 bral lobes pass into each jjost-cerebral ganglion so as to afibrd the means to the cerebral lobes 

 [primi inter pit res, as happily styled by Leydig) of coiirdinating the nervous power of the other ganglia, 

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

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

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

 us, thougli we are still open to conviction, that the transverse and longitudinal commissural tibers, 

 which und(jubtedly 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 difiieult 

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

 what little he has seen is, that the myeloid substance is tbe result of the splitting u]) into a tangled 

 mass of very tine fibrilhe of certain of the tibers thrown off from the mono-polar ganglion cells, i.e. 

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

 mind that in the embryo the ganglia are composed of ganglion cells alone, with few if any primitive 

 tibers. 



MORPHOLOGY OF THE BRAIN. 



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

 in the Decai)oda. It will be remembered that Professor Laukester in his memoir on Apus desig- 

 nates the suni)le brain of that crustacean as an archicercbrum, while the composite brain of " all 

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

 In our Monograph of N. A. Phyllopoda, p .403, we adojjted the view that the brains of all Crustacea 

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

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

 a ditt'erent jilan from that of the Decapoda. 



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

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

 sections, especially those represented by Pigs. 5-8, 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 

 u^sophagus, which of course is not rex>resented in tiie sketch; being so near the median line it 

 does not involve the optic lobes and eyes, which, especially the latter, ai'e on the extreme side of 

 the bodj', 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 ai>pendages are made obvious in the 

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

 cBsophagus open between the mandibles They are shown in Pig. 5. The end of one of the 

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

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

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



As seen in Pig. 1, the brain or supra(esophageal ganglion is a composite mass or group of 

 four i)airs of ganglia, /. c, (1) the brain proi)er or procerebral lobes, (2) the optic ganglia, (3) the 

 tirst antennal, and {4) the second antennal lobes. These lobes are quite separate from each other 

 in the Isopoda and Amphipoda as comi)ared with the Deuapoda. 



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 homologiies of tlie lobes of that name in Vertebrates, either 

 stiucturally or functionally, we would suggest that the ganglion be termed the i>rocerebrum and 

 the individual lobes the procerebral lobes, not only in allusion to its position in advance of all the 



