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1916-17.] Experiments and Observations on Crustacea. 
of Ligia — see Hewitt (1907) — each bundle of muscle fibres of this series 
arises immediately behind the anterior border of one segment and is 
inserted into the anterior border of the succeeding segment. The ventral 
longitudinal muscles, with attachments similar to those of the dorsal series, 
lie more laterally, where the sternites have greatest rigidity. From the 
anterior surface of each ventral inturning arise a number of small muscular 
bundles, which spread out radially to be inserted into the overlying inter- 
segmental cuticle ; these presumably contract during the act of extension 
of the body. 
The epimera are not flattened and imbricated like those of many 
Crustacea. In the immediately preceding paper of this series (p. 84), 
it was suggested that flattening and imbrication of pleural plates may give 
mechanical support and guidance during ventri- and dorsi-flexion. While 
the prevailing view is that they are designed for protection of underlying 
parts, one cannot believe that the whole story is therein told ; for when- 
ever we find flattening and imbrication of pleural plates in Crustacea, 
whether in perteon or in pleon, we discover the corresponding region to be 
movable. It is even possible that imbricating pleural plates subserve 
during movement a sensory as well as a mechanical function. In any case 
the hypothesis that specifically connects them with movement is more 
open to experiment than that which vaguely attributes to them a “ pro- 
tective ” function. 
In the case of Glyptonotus, which is peculiar in that many of the 
basipoclites in extension are visible from the dorsal aspect, it might be said 
that the existence of such free limb movement is incompatible with the 
presence of ventrally directed lateral plates, in other words, that protective 
structures have to be sacrificed in the interests of mobility ; but against 
this line of argument is the fact that they are equally absent in the three 
anterior segments, the basipodites of which remain hidden on dorsal view. 
Noting that the body of the animal as a whole is characterised by 
immobility rather than by mobility, one might say that the phenomena 
presented are at least not opposed to our hypothesis. 
As has been pointed out in all accounts of Glyptonotus , the line of 
junction between coxopodites and tergites is visible only in the case of 
segments 5, 6, and 7. Indeed, the distinction at present drawn between the 
antarctic genus Glyptonotus and the more widely distributed northern genus 
Chiridotea * lies in the number of epimera thus obviously separated from 
the thoracic tergites, Chiridotea showing a series of six separate epimera. 
* As a rule in this paper I shall use Harger’s term Chiridotea to include Miss Richardson’s 
two genera Chiridotea and Mesidotea. 
