122 DR. L. A. BORRADAILE ON THE 
the bod}' and upwards towards it. They arise from the endophragmal 
skeleton, and are inserted on the base of the scaphognathite. In its anterior 
part, the scaphognathite is crossed, at right angles to its long axis, by half-a- 
dozen bands of muscle, which, pulling downwards, curve its surface. These 
are the " accessory muscles." They arise from the sclerites of the basal 
portion of the limb, and are inserted on the anterior of those which support 
the scaplioguathite. A feeble strand of muscle runs across the base of the 
endopodite to the second cleft lobe, but independent movements of these 
structures, if they take place at all, are insignificant. 
The movement of the maxilla consists in a flapping of the scaphognathite 
to and 'from the roof of the exhalent passage of the gili-chamber. It is 
carried out in two ways, according as the current is being directed backward 
or forward. To drive the water forward, the posterior and outer end of the 
scaphognathite is smartly applied (presumably by the ''outer flexors") to 
the roof. Then the accessory muscles, which have been keeping the organ 
carved, allow the flexors to bring the remainder of it into the same position, 
with the undulating movement described by Garstang (8). Finally, it is 
drawn downwards by the extensors, being at the same time curved by the 
accessory muscles, so that it becomes concave towards the roof. To drive 
the water backward this procedure is reversed. The bending of the scapho- 
gnathite which it involves is facilitated by the fact tnat the two supporting 
sclerites of that organ are united by a flexible region. It seems possible 
that the function of the muscles of the coxa is to hold firm the protopodite 
when the accessory muscles contract. The lobes of the median edge of the 
limb may he seen to be drawn passively to and fro with each stroke of the 
scaphognathite. 
5. The Maivillule (First Maxilla) is a small limb, flattened and curved to 
fit against the surface of the mandible, and composed of three inwardly- 
directed lobes — the iuner and outer lacinise and the endopodite — and an 
external basal portion which unites them. Jt is usual to regard the lacinise as 
representing the coxa and basis, but, as I have elsewhere argued (5), the proxi- 
mal lacinia is probably the gnathobase, or endite of the true first segment of 
the Crustacean limbj "the 'precoxa' or 'pleuropodite,' which may or may not 
have originally existed as a free joint in every biramous limb, but has now 
nearly always disappeared, either by fusion with the trunk or with the second 
joint (coxa or coxopodite), or perhaps sometimes by excalation." In the 
maxilla it is represented by one of the components of the first cleft lobe. 
This lacinia is narrow and strong, and curves backward and dorsally to enter 
the mouth, at the hinder end of the mandible. Its base is widened and is 
continued across the face of the limb as a ridge, covered parti}' with thin 
cuticle but supported by a sclerite, and projecting anteriorly. Towards its 
outer end this ridge turns backwards (posteriorly), with a suture in the 
sclerite which supports it, and dies away upon the stout articular sclerite. 
