A STUDY IN CARCINOLOGY. 33 
the Eaiiinida?. He s:iys (34, p. 13G), "thoracis interior structura ofFert 
sellain turcicain nti taiuuiii in Brachyuiis, atque canalem sternalem iiti in 
MacTouris ceteriin tantum invenitur. Apodemata singula tarn sterni quani 
epiinerorum ab iitraque parte medio thorace jiinguntur in quoque articulo 
sternali, canalem sternalem constitnentia vertebrarnmque formani ])lane 
simnlantia. Apodemata ceplialica pariter adsnnt quin(iue transversa, ita ut 
Eaninw hoc respectu opposite sunt Dromiis, in quibus apodemata tlioracica 
et cejjhalica separatim versus centrum conmiune petunt." We have here 
a combination of Macruran and Brachyuran characters which, when the 
ancestry of the Raninidaa is in question, deservps close study. As a prelim- 
inary to this part of the subject, T must go in some little detail into the 
structure and mor['>holooy of the endophraginal skeleton. It was described 
both for Macriira and Braehyura by Milne Edwards (25), and since his 
day does not seem to have been studied in further detail. We have, sub- 
sequently, the brief but very accurate description of these structures in the 
Crayfish by Huxley (38). His figures, however, are not drawn from a point 
of view that clearly illustrates the structures described, and I shall endeavour, 
with the aid of new figures, to make clear some points that are necessary for 
a detailed comparison with the homologous structures in crabs. Fig. 11 is a 
ventro-lateral view of the sterna and the endophragmal skeleton of the second 
and third pereiopodal segments of the common Crayfish, seen f lom the left 
side ; the thorax has been tilted over to the right ; the outlines of the 
articular cavities of the limbs have been thickened to make them more con- 
spicuous. Ventrally we see the keel-shaped sternites, and at the outer side 
of the posterior end of each is a small articular socket, into which fits a 
corresponding knob on the coxa of the appropriate limb. Above are the 
corresponding epimera, separated from one another by a groove which is not 
a suture but a deep infolding of the cuticle, technically known as an apodeme. 
If we follow this groove down below the upper limits of the articular cavities 
of the limbs, we see that it bends forwards, then turns rather abruptly back- 
wards, and seems to end just Below a little knob which fits into a corre- 
sponding cavity on the coxa of the appropriate limb. Thus the articular 
knobs, which I shall call strophidia, are borne on prolongations of the anterior 
ends of the pleura, the articular cavities, which I shall call strophingiii, 
on the posterior ends of the sterna *. The line indicating the epimeral 
* These siuaU aiticular liiiobs and cavities have leen described in some detail, and their 
mechanical action has been described ly Herrick (37), but thoug-h often figured, they seem 
to have been neglected by other authors. As I sliall have occasion to refer to them 
frequently and to use them as landmarks, I have found it necessary to give them name?, and 
call the socket the strophidmm, the peg which fits into it the strophingium. The names are 
derived from the peg (crTpo<plyytov) and socket {a-rporpevi) hinge of a Greek door. For the 
correct formation of the diminutive arpocpevs, a-rpofjietScov, strophidium, I am indebted to 
Mr. Eric Barber of Exeter College. 
LINN. JOURN. ZOOLOGY, VOL. XXXV. 3 
