83 
1916-17.] Experiments and Observations on Crustacea. 
♦ 
evolution ? In the literature I have found two papers, those of Herrick 
(1905) and of Emmel (1906), in which a total torsion of 90 degrees is 
proved to occur within the length of the regenerating cheliped of the 
lobster. From these papers, however, one cannot say whether the torsion 
actually involves the individual segments or not. 
In this connection one might refer to the peculiar direction of dactylo- 
podites in the posterior thoracic limbs of some of the Ampeliscidm, Atylidse, 
and Gammaridse — see the illustrations in Sars (1895). In these animals 
the posterior dactylopodites hook in the same direction as the anterior. 
From a structural point of view it might seem a simple matter to reverse 
the direction of flexion of a dactylopodite (reversion of the direction of 
the carpo-propodal articulation can apparently occur — see the illustrations 
of Arcturus anna and cornutus in pi. xix of Beddard’s (1886) “ Challenger ” 
Report). From the point of view of physiological co-ordination of move- 
ment, reversion of the direction of a dactylopodite is not so simple, for it 
must be presumed to involve far-reaching adjustments in the central 
nervous system. To assume a torsion of the mero-carpo-propodal axis 
avoids the central nervous system difficulty, but fresh a priori difficulties 
then seem to arise in regard to the peripheral muscles. In the absence of 
appropriate observations on the living animal one cannot of course profit- 
ably pursue the subject, but the interest of such cases is very great. ( Gf ’. 
also the direction of the dactylopodite in the chela of Trischizostoma 
— Sars (1895), pi. xii.) 
To recur, however, to the peculiar orientation of the posterior thoracic 
limbs in the Gammaridea, one might offer the suggestion that the isopod 
Pkreatoicus progresses in a fashion similar to that common among the 
Amphipoda. In Chilton’s (1894) account there is little direct reference 
to this point, but his discussion of the general build of the animal is 
suggestive.* I here set down in his own words his list of amphipodan 
similarities : — 
“(1) The body, especially in the pleon, is more or less laterally 
compressed. 
“ (2) The pleura of the segments of the pleon are produced downwards, 
so as to protect the pleopoda on either side, just as in the Amphipoda. 
“ (3) The legs of the peraeon consist of an anterior series of four and 
a posterior series of three. 
“ (4) The general appearance of the legs and of the uropoda is not 
unlike that common among the Amphipoda. 
* On page 208 Chilton says : “ Phreatoicus walks erect or swims much in the same way 
as the Amphipoda.” 
