314 ME. W. B. S. BENHAM ON THE MUSCULAK 



Part II. Description of the Muscular and EndosJceletal Systems o/Limulus. 

 By W. B. S. Benham, B.Sc. 



Hard Parts. 



Some of the Hard Parts in Limulus to which Muscles are attached. 



I. The Tergites. 



a. External View. 



1. The Prosomatic Carapace has a horseshoe-shape, rounded and convex in front and 

 at the sides, which latter are produced beyond the central portion, ending in a point 

 behind. (For a general description and figures of the segments fused to form the 

 anterior and posterior carapaces of Limulus, see Lankester, "Limulus an Arachnid," 

 Quart. Journ. Micr. Sci. 1882.) 



The carapace is bounded behind by an almost straight line, reaching about halfway 

 on each side of the middle line. This straight portion bends sharply downwards, so 

 that the posterior border is nearly vertical, but of little depth. In the middle of this 

 portion is an arch, and on each side of this is a slight depression running forwards 

 along the carapace to about half its length : this depression produces a ridge on the 

 inner surface, at the posterior end of which an invagination of the chitin has taken 

 place, forming a pair of entapophyses (Pis. LXXIIL, LXXV., and LXXVL, Ent 1 ), 

 indicated externally by a shallow pit, on each side of this hinder arch, and situated in 

 the vertical border. 



Outside these two parallel depressions, about two thirds from the middle line to the 

 edge of the carapace, is a slight longitudinal ridge ; on this ridge are situated the 

 lateral eyes. It is outside this ridge that the carapace commences its downward 

 course. 



2. Meso-metasomatic Carapace.— The hinder border of the prosomatic carapace is 

 joined to the front edge of the abdominal (meso-metasomatic) carapace by a leathery 

 membrane extending right along the straight border ; on this border is an arch corre- 

 sponding to that in the prosomatic carapace. This front border is bent slightly 

 downwards, and at the end of the straight piece bends backwards and outwards, 

 parallel with the recurved portion of the sides of the prosomatic carapace. About 

 halfway along this oblique border is a ridge, ending in a point directed outwards. 

 At the edge the meso-metasomatic carapace is produced into six sharp recurved 

 points, between each consecutive pair of which is a rounded excavation in which is 

 articulated a movable spine : there are thus six pairs of movable spines to this 



