138 BRITISH FOSSIL CRUSTACEA. 



the entosternon (PI. XXXV, fig. 5 m, 3). 1 They tend to draw that part forward, and 

 resist the backward displacement of it, which would otherwise ensue in the action of the 

 1 depressores thoracetri' (m, 2). The 'pratrahentes entosterni' are the main origins or 

 'fixed points,' functionally, of the great muscles, made 'digastric' by the intervention of 

 the entosternal fibro- cartilage, which depress the thoracetron ; when the insertional 

 lamella of the ' depressores thoracetri ' become fixed points, they act through the medium 

 of the entosternon as origins of the digastric muscles deflecting the cephaletron. In like 

 manner, when the insertions of the ' levatores thoracetri ' become the fixed points or 

 origins, those muscles will oppose the ' depressores cephaletri' and become ' levatores ' of 

 that part. 



" Levatores antici sterni. — But the singular structure which acts functionally as 

 ' endoskeleton ' in Limulus has additional powers given to it by muscles which, like the 

 mainstay of a mast, steady it in the transverse or lateral directions. From near the fore 

 part of the dorsal surface of the entosternon diverge a pair of sclerous processes, which 

 become tendons of a pair of muscles (PI. XXXV, fig. 5 m 4) 2 about half the size 

 of the f protractores' (ib., fig. 5 m 3), and which have their fixed points in the antero- 

 lateral part of the cephaletron. The 'levatores,' by their direction, tend to raise and draw 

 forward the entosternon, and so add their power to the protractors when these muscles 

 are made to act in combination with the ' depressores thoracetri,' but to the degree in which 

 their oblique course would tend, if one of the pair acted singly, to pull the entosternon 

 sideways, their combined action would add to its fulcral power in relation to the move- 

 ments of the two chief divisions of the body. 



"Levatores laterales entosterni. — The steadying of the entosternon is more directly 

 attained by a series of fibres which, rising from ridges due to the inflection of the lateral 

 longitudinal grooves of the cephaletron, descend and converge to be inserted into the 

 posterior half of the lateral borders of the entosternon (ib., m 5). 



" Levatores postici entosterni. — Fasciculi from the dorsal surface (PI. XXXV, fig. 5, 

 m 6), which seem to be the fore part of the series of ' depressores thoracetri,' ascend, as 

 they retrograde, to be inserted into the lamelliform entapophyses rising from the hind 

 border of the cephaletron, which seem to initiate anteriorly the series of shorter and 

 smaller ones descending from the thoracetron. With the insertion, or rather origin, of 

 the above entosternal muscles, their action would be to retract and raise the ento- 

 sternon. 



" The functions of these ' levatores entosterni,' in relation to the fixation of the endo- 

 skeleton, are more especially in opposition to muscles arising from its lower and lateral parts 

 to be inserted into basal entapophyses of the five posterior pairs of cephaletral limbs. 

 But the principal muscles acting on the basal joints of these members arise from the 



1 J. vander Hoeven, ' Recherches sur 1'IIistoire naturelle et l'Anatomie des Limules,' fol. 1838, 

 p. 47, pi. iii, fig. lb, 



2 Ibid., fig. 7. 



