10 



The structiu'e of the teguments in Limulus polyphemus agrees with that in Idmuliis 

 mohiccamis *. 



§ 3. Muscular System. — The parts sent inward from the crust or exoskeleton are those 

 that afford attachment to muscles, and those which also form or contribute to the joints 

 of the articulate appendages. They are termed ' entapophyses ' and ' apodemes.' The 

 ' apodemes ' that relate to the cephaletral limbs (PL V. ii-vi) are broader and more 

 complex than those of the thoracetron (/&. vii-xiii). The most conspicuous entapophyses 

 are the following : — A pair of oblong lamelliform processes descend from the segment 

 confluent with and forming part of the hind border of the cephaletron at the parts 

 indicated by the ciliate depressions (PI. I. fig. 1, h). Six pairs of similar, but rather 

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

 the parts indicated by the oblong depressions (PI. I. figs. 1 and 2, i 1-6). 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. II a. fig. 3, 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 upon the thoracetron, and of the tail-spine upon the latter, and 

 reciprocally. 



The fixed points from Avhich 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 (PL II a. figs. 1 and 2, h) is situated partly in the angle between the 

 guUet and stomach, thence extending backward a short way along the interval be- 

 tween the beginning of the intestine and the neural axis. It is an oblong sub- 

 quadrate plate of sclerous or fibro-cartUaginous tissue, and is chiefly related to the 

 attachment of muscles (PL IV. fig. 5). It was likened by its discoverer, Stratjs Dukck- 

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

 terms in Arachiilda. I shall refer to it, without any wider homological signification, 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 medilateral grooves or inflections of the 

 carapace t- This feature in the accentuation of the upper crust of the cephaletron 

 relates to such favourable condition of origin of the ' levatores thoracetri.' The pair 

 come into contact at the median line, filling the hoUow of the roof, of which that line is 

 the mid ridge : their longitudinal fibres (PL II A. fig. 1, m i) 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 corresponding arch,/, 

 of the cephaletron. 



Depressores thoracetri. — The flexors or ' depressors ' of the thoracetron rise from the 



* Van der Hoeven, ut suprH, p. 15. 



t The corresponding grooves rendering Asaphus &c. ' trilobitic ' most probably indicate analogous ridges or 

 entapophyses for the flexor muscles of the segments. 



