MUSEUM OF COMPARATIVE ZOOLOGY. 263 



shown that those spines may stand either on ridges at right angles to the length 

 of the arm (Fig. 1, B C D) or on the outer edges of the side arm-plates, parallel 

 to it (Fig. 1, A E). In the former case the wider diameters of the spines are 

 upward, like the paddles on a wheel {Ophiothrix) ; in the latter, the spine has its 

 edge upward as if it had heen revolved 90° on its own axis. The variations in 

 these little organs are almost endless ; extremely long, thin, glassy, and thorny 

 [Ophiothrix Sucnsonii), very small, thick, opaque, and smooth (Ophiomusium 

 iburneum) ; rounded and tapering (Ophiocnemis m.armorata) ; flat and of even width 

 {Opliiura cinerea); covered with thick skin {Ophiomyxa flaccida) ; naked (Ophio- 

 coma echinata); club-ended {Ophiomastix annulosa); armed with hooks (under 

 spine in some species of Ophiothrix). The importance of the arm-spines depends 

 partly on their length, thickness, and number, and partly on the extent of the side 

 arm-plates. When these last occupy a large part of the circumference, and the 

 spines also are long and numerous, the arm proper is almost hidden and resembles 

 a round bristle-brush {Ophiomyces frutectosus). 



q. Papillce ambulacrales ; tentakel schuppen ; papilles tentaculaires ; tentacle- 

 scales. Although these small organs are strictly homologous with, and even some- 

 times similar to, the arm-spines {Ophioglypha), they nevertheless have a different 

 function, to wit, that of covering the tentacle drawn in ; and usually they differ 

 in form and position from the nearest arm-spine. Thus in Ophiothrix, while the 

 lowest spine is often hooked, the tentacle-scale is minute and tooth-like ; in 

 Ophiopsila it is like a spatula ; Ophionereis has a single circular one ; Ophiocoma 

 one, or two, of a shape more or less oval. In Opliiura the upper scale laps over 

 the base of the lowest arm-spine, though in most genera the two are separated. 

 One of the tentacle-scales is often carried by the under arm-plate, either on its lat- 

 eral edge {Amphiura) or on its surface, making a continuation of the line of arm- 

 spines {Ophiomyces). Although some species of Amphiura have two scales, one 

 on the edge of the under arm-plate and the other at right angles on the edge of 

 the side arm-plate, others have no scale at all, — a want shared by many species 

 {Ophioscolcx ylacialis, Ophiomyxa, Ophiopsammium). The two pairs of mouth- 

 tentacles are not neglected in this respect (Figs. 3, 4, q' q") ; but, in almost all 

 genera, are furnished with one or more tentacle-scales (q q). That of the first, 

 or upper, tentacle sits on a little ridge of the jaw. It is this, when largely devel- 

 oped (as in some species of Amphiura), that has been described as a peculiar 

 papilla situated high up in the mouth. Ophiothrix is one of the few genera that 

 lacks this scale, which exists even in Ophiomyxa. While these scales are closely 

 homologous, on the one side, with arm-spines, they also are, on the other, with 

 certain mouth-papilla?. Of this there is an illustration in Ophioglypha (Fig. 4), 

 where the tentacle has a row of scales on either side of it, one row carried on the 

 edge of the side arm -plate (i), the other on that of the under arm -plate (some- 

 what displaced in the figure). It is a greater development of what occurs in the 

 Amphiura: with two scales (PI. IV. Fig. 20). The homologue of the side arm- 

 plate which belongs to the innermost under plate is the side mouth-shield ; and 



