718 DB. W. B. CAHPENTEE ON THE STRUCTUKE, PHYSIOLOGY, AND 



transverse to the axis of the arras, forming what is called a syzygy [sg), — a peculiarity 

 which will be presently more fully described (§ 50), Excepting where syzygies occur, 

 each segment after the first bears an articulated pinnule on its wider margin ; and thus 

 the pinnules spring alternately from each margin of the arm, the total of the two series 

 equalling in number that of the segments, the pair forming each syzygy being counted 

 as one. The articular facets for the basal segments of the pinnules, which are seen 

 when the Arms are viewed on their lateral or their ventral aspects (Plate XXXVII. 

 fig. 4, p, 2>), each consist of a shallow socket divided by a transverse ridge, which is per- 

 forated by a minute aperture. In this basal portion of the arms, the articular facet of 

 the pinnule encroaches on the distal articular surface of each segment ; but in the 

 Middle and Terminal portions of the Arms, we shall find that in consequence of the 

 change of relative dimensions of the segments, the pinnules are articulated to their 

 lateral faces. 



42. Even on the dorsal aspect of the Arms (Plate XXXVII. fig. 3), we notice that 

 the margins of the successive segments do not come into close approximation except at 

 the syzygies (sff, sg); and the spaces between them are occupied by an elastic ligamentous 

 substance, which antagonizes the flexor muscles. These muscles (Plate XXXVIII. fig. 

 10) occupy the large spaces which are seen on the ventral aspect of the Anns, when the 

 superficial soft parts have been cleared away (Plate XXXVII. fig. 4), to lie between 

 the apposed faces of the segments ; each segment being thinned away towards its upper 

 or ventral margin into a vertical plate, from the middle of which projects on either side 

 a sort of keel that fonns a buttress and divides each of the deep fossae left between the 

 successive plates into two lateral halves. Where a syzygy occurs [sg, sg), the vertical 

 plates of the two segments that form it come, into close contact, and the keel projects 

 only from one face of each, the two apposed segments thus taking the place of the single 

 segment elsewhere. This peculiar contrast between the dorsal and the ventral aspects of 

 the Arms is still better brought into view by a vertical section taken a little on one side 

 of the axis of the arm, so as not to pass through the projecting keels that divide the 

 muscular fossae, and only partially to lay open the axial canal (Plate XXXVIII. fig. 11). 

 This shows that the successive segments really come into contact with each other only 

 by the great transverse ridges which cross their articular surfaces (Plate XXXVI. 

 figs. 4-8, a, a), and embrace the opening of the axial canal ; and that the space between 

 these ridges and the dorsal surface, which is occupied by the elastic ligament, is much 

 larger than it appears externally to be. Of the large space between the axial canal and 

 the ventral margin, the part nearest the canal is occupied by the interarticular liga- 

 ments which are lodged in the shallow fossaj' that are seen on each articular surface just 

 above the transverse ridge; whilst the much longer spaces intervening between the 

 vertical plates are entirely filled by muscular substance {m, m), the fibres passing directly 

 from each plate to the next in front and behind, except in the case of a syzygy. 



' These are termed "articular facet ?" by Professor MCllek; but I feel satisfied that they cannot come into 

 contact Avith those of the succeeding segment, and that they have the character above assigned to them. 



