PROFESSOR OWEN ON LIMULUS POLYPHEMUS. 187 



processes project into the cavity of the thoracetron, from the inner surface of the parts 

 indicated by the oblong depressions. These serve to give attachment to and augment 

 the force of muscles. Analogous entapophyses are developed in most of the articulations 

 of the limbs (PI. XXXIV, fig. 5 c, e, g) for a like purpose. All these internal processes 

 assume more or less of a cartilaginous character, losing the hardness and colour of the 

 outer crust as they extend inwards. 



" The main movements of Limulus in locomotion are those of inflection and extension 

 of the ' cephaletron n upon the thoracetron, 1 and of the tail-spine upon the latter, and 

 reciprocally. 



" The fixed points upon which the cephaletral muscles act upon the thoracetron are 

 afforded not only by the apodemata and entapophyses, but also by the representative of 

 an internal skeleton. This (PI. XXXIV, figs. 1 and 2 h) is situated partly in the angles 

 between the gullet and the stomach, thence extending backward a short way along the 

 interval between the beginning of the intestine and the neural axis. It is an oblong, 

 subquadrate plate of sclerous or fibro-cartilaginous tissue, and is chiefly related to the 

 attachment of muscles (PI. XXXV, fig. 5). It was likened by its discoverer, Straus 

 Durckheim, to an internal cartilaginous sternum, and may answer to the part which he 

 so terms in the Arachnida. I shall refer to it, without any wider homological signifi- 

 cation, as the 'entosternon.' 



" Levatores thoracetri. — The extensors or, more properly, ' levators ' of the thoracetron 

 are a pair of powerful muscles, the fibres of which rise from the low inner ridges indicated 

 or formed by the longitudinal mediilateral grooves or inflections of the carapace. 2 This 

 feature in the accentuation of the upper crust of the ' cephaletron ' relates to such 

 favorable condition of origin of the • levatores thoracetri.' The pair come into contact at 

 the median line, filling the hollow of the roof, of which that line is the mid-ridge ; then- 

 longitudinal fibres (PI. XXXIV, fig. 1 m, 1) intervene between it and the pericardium as 

 they pass backward to be inserted into the anterior and upper transversely convex process 

 of the ' thoracetron ' (g), which enters the correspond arch (/) of the ' cephaletron.' 



" Depressor es thoracetri. — The flexors or ' depressors ' of the ' thoracetron ' rise from 

 the dorsal surface of the hinder third of the entosternon (PI. XXXV, 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, to be inserted into the entapophyses 

 of the thoracetron. 



" Prtstrahentes 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 



1 See Note 2, page 186. 



2 The corresponding grooves rendering Asaphus, &c., "trilobitic" most probably indicate analogous 

 ridges or entapophyses for the flexor muscles of the segments. 



