740 DE. AV. B. C-IEPEKTEE OX THE STRUCTUEE, PHYSIOLOGY, AND 



period, depart even less than the preceding from the simple type of conformation which 

 they present at their first development (§ 67), being still obviously composed of the 

 original cribriform plate, whicli constitutes their convex dorsal surface, with an ingrowth 

 of fasciculated tissue that closes over the axial canal, and gives to the ventral aspect a 

 flattened sui-face with a median groove. This ingrowth is most abundant towards the 

 extremities, where also the meshes of the reticulation are smaller, so that these parts 

 have a more solid character than the rest. We now begin to see that alternate inclination 

 to one side and the other, which is so marked a feature in the articular extremities of 

 the segments of the adult ; and we also notice that whilst these extremities are in close 

 contact with one another on the dorsal aspect (b), except where a small depression exists 

 in each for the attachment of the articular ligament, the articular surf;xces of the ventral 

 face (.\) slope away from each other so as to leave a considerable space for the lodgment 

 of the two muscular bundles, which are now conspicuously interposed between each 

 pair of segments in the basal part of the arms, with certain exceptions. These excep- 

 tions correspond with those which present themselves in the adult (§i^ 47-50) ; though the 

 peculiarities of conformation which mark them are as yet but little pronounced. Thus 

 the adjacent extremities of the first and second brachial segments present nearly the 

 same aspect on their ventral as on their dorsal face, no bevelling-off being seen on the 

 articular surfaces of either ; and they are connected by ligament only, no muscular fibres 

 being here distinguishable. Again, the adjacent articular surfaces of the third and 

 fourth segments come into yet closer contact, not even a connecting ligament being 

 interposed, and the line of their junction being transverse instead of oblique ; this pre- 

 figures the peculiar syzygal union which shows itself between these and other pairs of 

 segments in the adult (§ 50). — Towards the extremities of the Arms, we find the seg- 

 ments even more cylindrical (Plate XLI. fig. 4, A, B), except where lateral processes are 

 given off for the articulation of the pinnules ; and their terminal faces are simply apposed 

 to each other transversely, without either the alternate obliquity or the bevelled articular 

 surfaces of the basal segments. And at the growing points of the Arms precisely the 

 same rudimental condition of the segments presents itself (Plate XXXVIII. fig. 1), 

 as that which has been already described [^ 67) as showing itself at an earlier period 

 in their basal segments. Thus we may trace in one and the same Arm several successive 

 stages of development of the pieces of its skeleton ; the most advanced segments (those 

 nearest the base of the arms) showing adumbrations of their adult peculiarities, although 

 not yet departing in any considerable degree from their simple primitive type. 



81. Besides the regular skeleton of the Arms, we commonly find their perisomc of 

 condensed sarcode to contain irregular branching spicules (Plate XLII. fig. 5, a,b), forming 

 a sort of incomplete reticulation for the support of the elevated folds which constitute 

 the margins of the ventral furrows. These are obviously the rudiments of the dermal 

 plates which have been shown by Professor J. Mullfk' to form a complete armour to 

 the ventral perisome and its prolongations along the ventral surface of the arms in Pcn- 



' Ober dci; Ban dos Pcntacnnus t'aimt-Mcdusm, p. 48, 



