No. I.] STUDIES ON LIMULUS. 15 



middle of the plastron, thus forming a pair of very large ante- 

 rior cornua {a.c, Figs, ii and 12). 



The body of the plastron may be considered as a plate of 

 cartilage with crenate margins and concave on the neural side. 

 Radiating from a common center on the neural side are four 

 paired, plate-shaped processes {n.pl.^~^, Fig. 11), and one poste- 

 rior unpaired one {n.pl.S, Fig. 1 1). The posterior one is thinner 

 than the others and gradually tapers out, ending in three low 

 ridges. The anterior pair borders the inner neural margins of 

 the anterior cornua {a.c). From about the middle of the an- 

 terior cornua spring a pair of processes {n.pr., Figs. 1 1 and 12), 

 which bend around the brain and attach themselves to the 

 integument close together on the neural side. 



On the haemal side of the plastron two high ridges converge 

 from the distal ends of the anterior cornua to the posterior end 

 of the plastron, forming a deep gully between them, in which 

 lies the alimentai^y tract. These ridges are split up into five 

 paired haemal processes {h.pr.i-~S, Fig. 12) of unequal length, 

 those in the middle being longer than those at either end. 

 The plastron ends posteriorly in a short median process {ppr., 

 Figs. 1 1 and 12). 



The muscles arising from the plastron are too numerous and 

 complicated to allow of a full description in this paper. From 

 nearly the whole of the neural side muscles go to the legs ; 

 haemo-neural muscles are attached to the haemal processes 

 (//./r.1-5), and longitudinal muscles to the posterior process 

 {p.pr.). 



The brain lies just haemal to the neural processes {n.pr.), 

 which are in contact with the integument. The oesophagus 

 passes through the brain and between the anterior cornua to 

 the sucking stomach, which lies in the groove on the haemal 

 side of the plastron. Muscle strands run from the stomach to 

 the walls of the groove. ^ ^ Redenbaugh. 



There are, therefore, four distinct structures in arachnids 

 that may serve as parts of a true endoskeleton : 



I. The endosternite or etido cranium, a broad flat plate of 

 fibro-cartilage lying on the dorsal side of the brain, and serving 



