THE ENDOCRANIUM OF LIMULUS. 



315 



posterior end. The brain lies on the floor of this box, the ventral cord and several 

 pairs of nerves extending backward through the large occipital foramen in the 

 posterior wall. (Fig. 214.) 



We may recognize the following parts, viz. The anterior cornua, a.c, a pair 

 of stout, transversely flattened processes formed by the forward prolongation of 

 the thickened lateral margins. From each process arise three muscles, the oppo- 

 site ends of which are attached to the hasmal side of the carapace (Fig. 75) ; one 

 is directed forward from the extremity of the cornua, one perpendicularly from 

 its inner surface, and one obliquely forward from its hsemal margin. 



The neural margins of the cornua and the entire lateral portions of the endo- 

 cranium, including the posterior lateral processes, give attachment to the plastro- 

 coxal muscles of the second to the sixth pair of thoracic appendages. (Fig. 75.) 



Anteriorly, the muscles do not cover the 

 neural surface of the endocranium, but pos- 

 teriorly the muscles increase in size with the 

 increase in size of the appendages, and encroach 

 upon the neural surface even to the median 

 line. There is therefore on the anterior neural 

 surface of the endocranium a triangular space 

 which, except for a few loose strands (plastro- 

 buccal muscles going to the oesophagus) is free 

 from muscles and comparatively smooth. (Fig. 

 215,4.) 



The anterior hczmal processes, I.e., arise from 

 the anterior haemal side of the endocranium. 

 They consist of two pairs of long and slender 

 processes each one attached by a short muscle 

 to the haemal side of the carapace, close to the 

 origins of the tergo-coxal muscles. 



The posterior h<,mal proeesses, h.pr., lie on ,.JZ'::;J^'::^^^:^;Z:. 

 the haemal side near the lateral edge of the 



endocranium. They incline slightly outward, and each gives attachment to two 

 muscles: one going from the extremity of the process to the carapace, and the 

 other from the posterior margin of the process to the first entapophysis. (Fig. 75.) 



The posterior lateral processes, Ip.pr. , are flattened expansions of the posterior 

 portion of the endocranium. Along the posterior margin of each process, on the 

 neural side, is a sharp transverse ridge which, toward the median line, unites with 

 the lateral ridge and with the base of the occipital ring. The posterior-lateral 

 processes give attachment to some of the plastro-coxal muscles of the sixth pair 

 of legs, which are the most powerful appendages of the animal. 



The posterior median process, p.pr., or basioccipital, begins as a median ridge 

 on the haemal side of the endocranium, between the hsemal processes. It in- 

 creases in thickness posteriorly, ending in a bifid process, each division of which is 



