44 WILLIAM PATTEN. 



are its various cavities^ its cerebral hemispheres, and its infun- 

 dibulum. Although some of these parts can be identified with 

 a fair degree of certainty in insects, Myriapods, and Arach- 

 nids, the whole appearance of the fore-brain region in the 

 adult Limulus is totally unlike that of any other known Ar- 

 thropod. On the other hand, it bears such a striking resem- 

 blance to a Vertebrate brain that I believe no competent 

 person need hesitate u moment in picking out the correspond- 

 ing parts. 



' Before one can understand the structure and development of 

 the brain of Limulus one must first have a clear idea of its 

 structure in those Arthropods in which the cephalic lobes are 

 present in their simplest and most primitive condition. I 

 shall therefore first call attention to certain features of the 

 brain of Myriapods, Insects, and Arachnids, that are probably 

 common to the brain of all Arthropods. I have not paid 

 special attention to the Crustacea, and I do not, unless ex- 

 pressly stated, include them in my speculations on the Arthro- 

 pod brain. 



By a comparative study of the cephalic lobes of Acilius, 

 Blatta, Vespa, Hydrophilus, Scorpions, several species of 

 Spiders, and Limulus, specially prepared to bring out surface 

 contours, I am able to demonstrate that the cephalic lobes 

 of a typical Arthropod are composed of three distinct seg- 

 ments, each containing a segment of the brain, optic ganglion, 

 and optic plate; between the two latter is an invagination by 

 means of which the optic ganglia are more or less impeded. 

 These results have been in part confirmed by Wheeler and 

 Heider. Viallanes, who has made a careful study of the anatomy 

 and physiology of the adult brain, has quite a difi'erent con- 

 ception of its structure. Embryological studies, however, do not 

 support his views ; they show that each of his three segments, 

 protocerebron, deutocerebron, and tritocerebron, comprise very 

 heterogeneous centres, not at all arranged according to their 

 morphological affinities. St. Kemy, in his valuable work on 

 the brain of Myriapods and Arachnids, has followed Viallanes. 

 For lack of space I cannot review the works of these two 



