OF THE AMERICAN KING-CRAB. 469 
transversely convex process of the thoracetron, g, which enters the corresponding arch, f, 
of the cephaletron. 
Depressores thoracetri.—The flexors or * depressors' of the thoracetron rise from the 
dorsal surface of the hinder third of the entosternum (Pl. XXXVIII. fig. 5, m2), divide 
as they pass baekward into two groups, or a pair, the fibres of which ascend obliquely on 
each side the intestine, and subdivide into fasciculi (Pl. XXXVI. fig. 1, £, t), to be inserted 
into the entapophyses of the thoracetron. 
Pretrahentes entosterni.—The power of the * entosternon ' as a fixed point or fulerum 
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. XXXVIII. fig. 5, ms)*. They tend to draw that part forward, and 
resist the backward displacement of it, which would otherwise ensue in the action of the 
* depressores thoracetri’ (m2). The ‘ prætrahentes 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 
lamellee 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 cephaletrij and become *levatores' of 
that part. 
Levatores antici sterm.—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 (Pl. XX XVIII. fig. 5, m 4)f, about half the size of 
the * protractores ’ (ib. fig. 5, ms) and which have their fixed 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 fuleral 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 the 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 (Pl. XXXVIII. 
fig. 5, ms), 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, 
* Van der Hoeven, op. cit. p. 47, pl. ii. fig. 7, B. t Ibid. fig. 7. 
