200 WAGENER^ ON GYRODACTYLUS ELEGANS. 



form of an italic S, so as to inclose a heart-shaped space, the 

 attached base of the apron-like expansion just described is 

 necessarily wider than the free margiiij which reaches almost 

 to the point where the hooks begin to curve outwards. 



The lower appendage is very narrow ; its inferior border is 

 produced into a very thin membranous fold, whose lower 

 margin is slightly incised in a crescentic form. This appendage 

 resembles a short curved thread or wire, resting on its free 

 slightly ascending extremities on both hooks. The central 

 disc presents a V-shaped transverse section, which towards 

 the bases of the hooks expands in a single plane. 



The peripheral portion of the caudal disc is very motile. 

 At the periphery, except at the upper border, are placed six- 

 teen minute booklets, at equal distances apart ; and according 

 to the protrusion and retraction of these organs, the border 

 of the disc varies much in appearance. When the booklets, 

 together with the fleshy substance in which they are imbedded, 

 are protruded in a finger-like form, the intervals between them 

 are often considerably retracted, each interval exhibiting two 

 uniform incisions. On the other hand, Avhen the booklets 

 are strongly retracted, the intervals appear to be slightly 

 emarginate. 



Each booklet is individually motile. It may be retracted 

 deeply into the finely striated fleshy envelope with Avhich it 

 is encompassed, so that even the very point is concealed ; or 

 it may be protruded to such an extent that the whole of the 

 booklet is exposed, resembling a finger armed with a claw. 



In each of these little booklets three portions may be dis- 

 tinguished — the booklet itself, its stem, and an appendage, 

 of which parts the latter two are subservient only to the 

 motions of the organ. 



The booklet is flattened, strongly bent on the edge, and 

 very sharp. The base is produced before and behind into 

 two short wings, which are also placed upon the edge ; the 

 hook portion is seated upon the narrowest spot of this biscuit- 

 shaped figure. One of the wings in question lies on the 

 dorsal, and the other on the ventral aspect of the disc. 



To the latter wing is attached, in a manner not yet ascer- 

 tained, a very slender, elastic peduncle or stem, eight times 

 as long as the hook, and slightly knobbed at the free ex- 

 tremity. Though often much bent by the contraction of the 

 caudal disc, it quickly and readily straightens itself again on 

 the cessation of the movement. 



The wing corresponding to the under side of the disc serves 

 as the point of attachment to a faintly defined elongated 

 appendage, which is about half as long as the stem, and has 



