160 Macteay Memoria Vouvume. 
The exoskeleton of P. edwardszz has been described somewhat fully by one of us,* 
and the description will be referred to whenever it may be necessary to describe the 
relations of the muscles and aponeuroses to the hard parts. 
An interesting result of our observations is to show the incorrectness of the 
commonly received opinions that the whole of the great internal mass of abdominal 
muscles consists of flexors. Milne-Edwards’ account of the abdominal muscles of 
Homarus is still the standard authority on the subject, and is followed in all the 
text-books.{ As, however, we hope to show, he overlooked the fact that certain of 
what he called “muscles fléchisseurs” really act as extensors. That this is not a 
peculiarity of the Loricata, we have proved by an examination of Pavanephrops in 
which the general structure is so similar to that of the other Astacide as to warrant 
the assumption that there is no fundamental difference in the muscular system. 
Another point of considerable morphological interest is the discovery of certain 
vestigial muscles connected with the fused and immovable coxopodites of the antenne. 
I. Tae Axtat Moscties. 
1. Muscles of the abdomen. 
It is convenient to describe the whole mass of abdominal muscles as consisting 
of a dorsal, a ventral and a lateral set : the dorsal muscles are purely extensors, the 
ventral act partly as flexors, partly as extensors, the lateral as rotators. 
The dorsal muscles (figs. 1 and 10) consist of a superficial and a deep set of 
extensors. 
The superficial extensors consist of delicate sheets of muscle lying immediately 
beneath the dorsal integument of each segment, and passing in a longitudinal 
direction from the anterior border of each tergum to the anterior border of its 
successor. The muscle of each segment consists of two portions on each side of the 
middle line ; the fibres of the mesial portion are straight, those of the lateral portion 
slightly oblique. 
The superficial extensors of the first abdominal segment arise from the tergal 
region of the carapace immediately posterior to the heart. In the sixth segment 
these muscles are absent. 
* Parker, ‘‘On the Skeletons of the N.Z. Crayfishes.” Wellington (Col. Mus. and Geol. Sury. Dept.), 1889. 
+ H. Milne-Edwards, ‘‘ Hist. Nat. des Crustacés,” 1834, p. 151, Pl. x1. 
~ See, inter alia, Huxley, ‘‘ The Crayfish,” p. 99; Howes, ‘‘Atlas of Pract. Elem. Biology,” Pl. vim. fig. 24; Huxley 
and Martin, ‘‘ Pract. Biology,” Revised Ed. p. 205; Lang, ‘‘ Text-book of Comp. Anat.” Vol. I. p. 331 ; Lemoine, Ann. 
des Sci. Nat. Zool. 5™e série, T. IX. p, 228. 
