258 
Proceedings of the Royal Society of Edinburgh. [Sess. 
existence might warn one against designating any and every newly dis- 
covered calcification in a crustacean limb as a remnant of a primitive 
segment — see later under Pleopods, p. 273. 
Owing to the ease with which crustacean muscles become detached from 
their origin, and also owing to the brittle condition of the muscular fibres 
in the preserved specimens, considerable difficulty was experienced in 
determining the site of origin of the individual muscles. The following 
details, however, may be taken as reliable — see fig. 6. The posterior flexor 
arises near the mid-dorsal ridge from a narrow longitudinal area just 
lateral to the line of the dorsal longitudinal muscles of the trunk (see p. 265). 
Fig. 6. — Outline drawing of a thoracic tergite of Glyptonotus ; 
from dorsal aspect. To show the origins of the limb 
muscles ; cf. fig. 8. Natural size. 
a, origin of posterior flexor ; b, origin of anterior 
flexor ; c, origin of anterior extensor ; d, origin of posterior 
extensor. 
The posterior extensor arises by a transverse origin from the posterior 
edge of each sculptured triangle on its own side of the tergite. The 
anterior extensor arises from an extensive transverse area in front of this. 
The anterior flexor arises from another transverse area still farther 
forward, the greater part of its fibres being derived from a ventral inturn- 
ing of the anterior border of the somite. It is worthy of mention that 
no muscular fibres are derived from the interior of the coxopodite. All the 
muscles operating a given basipodite spring exclusively from the tergite 
corresponding to that basipodite. 
The Perceopods. 
Most figures of Glyptonotus (that by Dr Bruce is an exception) show 
the peraeopods directed posteriorly in the line of the body axis, the dacty- 
lopodites even of the first pair being situated behind the fifth (free) thoracic 
segment. It can be readily shown that the centre of gravity of the 
extended body lies within the limits of this segment. Consequently, unless 
articular membrane against nipping between the bearing surfaces, with consequent greater 
freedom of movement at the joint. Only the articular membrane in the neighbourhood of 
the condyles incurs risk of nipping ; that on the medial aspect of the joint, in relation to 
the tendon of the posterior flexor, is not liable to be nipped. 
