468 PROF. OWEN ON THE ANATOMY 
developed within the semicircular lower excavation, receiving the condylar part of the 
base of the tail-spine. 
The structure of the teguments in Limulus polyphemus agrees with that in Limulus 
moluccanus *. 
$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. XXXIX. 11-v1) are broader and 
more complex than those of the thoracetron (ib. vir-xiir). The most conspicuous 
entapophyses are the following :—4A 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 (Pl. XXXVI. fig. 1, 4). 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 (Pl. XXXVI. 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 (Pl. 
XXXVII. fig. 3, c, e, y) 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 which 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. XXXVII. figs. 1 and 2, A) is situated partly in the angle between 
the gullet and stomach, thence extending backward a short way along the interval 
between the beginning of the intestine and the neural axis. It is an oblong sub- 
quadrate plate of sclerous or fibro-cartilaginous tissue, and is chiefly related to the - 
attachment of muscles (Pl. XX XVIII. fig. 5). It was likened by its discoverer, STRAUS 
DtURCKHEIM, to an internal cartilaginous sternum, and may answer to the part which he 
_so terms in Arachnida. 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 museles, the fibres of which rise from the low inner ridges 
indieated or formed by the longitudinal mediilateral grooves or inflections of the 
carapace +. 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 hollow of the roof, of which that line is 
the mid ridge: their longitudinal fibres (Pl. XXXVII. fig. 1, m1) intervene between it 
and the pericardium, as they pass backward to be inserted into the anterior and upper 
* Van der Hoeven, ut supra, p. 15. 
t The corresponding grooves rendering Asaphus &c. * trilobitie' most probably indicate analogous ridges or 
entapophyses for the flexor muscles of the segments. 
