262 BULLETIN OF THE 



reduced to small bead-like projections. Then there are two different forms, — 

 the ridge, which stands out free of its neighbors, and bears the spines nearly 

 at right angles to the axis of the arm (Fig. 1, B D), (Ophiocoma, Ophiothrix, 

 Ophiacantha) ; and the flat, which clings close to the surface and overlaps the 

 next plate beyond (Fig. 1, A E ; Fig. 2, A I), (Ophiura, Ophioglypha, Pccti- 

 nura). ' These bear the spines on their outer edge, and lying close to and par- 

 allel with the arm. In consequence of two such modes of structure, Ophiurans 

 are divided into supple-armed and stiff-armed. The ridge-like plates give space 

 for a more or less free lateral motion, while the flat and overlapping ones impart 

 rigidity. If a living Ophiothrix be placed side by side with an Ophiura, the 

 former will wriggle briskly along the bottom, while the latter may lie quite torpid, 

 or only slightly bend its rigid arms. Dr. Graeffe told me that one of the most 

 singular spectacles he had seen was an Ophiothrix longipeda swimming free, and 

 with its five immensely long arms in rapid and perplexing motion. Ophioglypha 

 has more mobility than Ophiura, but its way of lifting itself along by two of its 

 arms, as described by Professor Mobius,*' is very different from the lively squirm- 

 ing of the arms of an 0})hiothrix. Even in genera which are clothed by a thick 

 skin, such as Ophiomyxa, the side arm-plates will be found well developed under- 

 neath. 



j. Scutella dorsal ia ; riickenschilder ; plaques dorsales du bras; upper arm- 

 plates. Like those of the lower surface, they may either remain very small, as 

 when they first appear at the arm-tip (Ophiomusium, Fig. 2, I/), or may develop 

 into wide scales, covering the whole upper surface (Ophiura, Fig. 2, A/; Ophi- 

 arachna). It is these plates that are specially liable to multiplication, either by 

 breaking up mechanically, or by the addition, in various ways, of supplementary 

 pieces (see p. 267). The extremest case is furnished by Eemieuryale, where the 

 original plate becomes lost in a mosaic of additional pieces (Fig. 2, H j). 



I. Scutella radialia ; radialschilder ; plaques radiales ; radial shields. Not 

 properly a part of the general scale covering of the disk, and belonging rather to 

 the interior skeleton through their connection with the genital plate, these shields 

 are an exceptional feature, and one that never is wanting, although sometimes hid- 

 den by thick skin {Ophiomyxa, Fig. 2, C), or by scales and granules (Ophiura, 

 Fig. 2, A) ; sometimes very large, with their inner portion buried by scales (Ophi- 

 othrix, Fig. 2, D; Ophioglypha, Fig. 2, E), or, again small and narrow (Amphi- 

 ura, Fig. 2, F). 



771. Radial scales sometimes exist as large scales just next the outer end of the 

 radial shield. In OphioaJypha they serve to support an arm-comb of small papil- 

 lae (Fig. 2, E m). 



n. Genital scales are largely developed in some genera, where they bound a part 

 of the body wall of the genital opening (Ophioglypha, Fig. 1, E n), and even 

 pass upwards and arch over the base of the arm (Ophiothrix, Fig. 1, D n). 



p. Spina; brachialcs; arm stacheln ; piquants ; arm-spines. It has already been 



* Schriften des Naturw. Vcreins fur Schleswig-Holstein, 1. 179, "1873. 



