11 



dorsal surface of the hinder third of the entostemum (PL IV, fig, 5, m 2), divide as they 

 pass backward into two groups, or a pair, the fibres of which ascend obliquely on each 

 side the intestine, and subdivide into fasciculi (PL I, fig. 1, t, t), to be inserted into the 

 entapophyses of the thoracetron, 



Pi'CBtraheutes entosterni. — The power of the ' entosternon ' as a fixed point or fulcrum 

 is provided for by other muscles, A strong longitudinal subdepressed fasciculus rises 

 from the inner surface of the fore part of the cephaletron on each side, the fibres of which 

 slightly converge as they pass backward to be inserted into the anterior angles of the 

 entosternon (PL IV, fig. 5, m 3) *. They tend to di-aw that part forward, and resist 

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

 ' depressores thoracetri ' {m 2). The ' prsetraheutes 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 

 lamellae of the ' depressores thoracetri ' become fijsed 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 j)owers given to it by muscles which, like the 

 mainstays 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 (PL IV. fig, 5, m i)\, about half the size of 

 the ' protractores ' {ib. fig, 5, m 3) and which have their fix:ed points in the antero-lateral 

 parts of the cephaletron. The ' levatores,' by their direction, tend to raise and draw for- 

 ward 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 directlv 

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

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

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



Levatores postici entosterni. — Pasciculi from the dorsal surface (PL IV. fig, 5, nn), 

 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, wiiich 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 

 entosternon. 



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



* Van der Hoeven, op. cit. p. 47, pi. iii. fig. 7, b. f Ibid. fig. 7. 



c2 



