Se en ee een ie MES LR Te Per me SU Ee 
My Se ee aed ee ee ee ee 
C. F. Liitken on Ophiurans. 57 
as a general rule, occupy the whole upper surface of the arm, 
but the under plates may be square or eight-sided, and are often 
cut out on the sides to give room for the tentacle scales. The 
innermost under plate varies in shape, and is often very small. 
At the extremity of the arm the joints are proportionately longer 
and are contracted at their bases; the upper and under plates 
become smaller and are supplanted by the side plates, which 
meet on the middle lines above and below, and at last constitute 
almost the whole covering of the tip joints. Therefore, the 
shape of the plates, exposed as it is to constant changes, shou 
always be referred to the portions of the arm close to the disc. 
These modifications appear sooner in species with short and 
quickly tapering arms, than in those with longer and more slen- 
der ones. There are, however, many serpent-stars, the inner 
ng to Gaudry, the four rows of little bony 
a of the arm and under the skin, among 
uryalz, correspond to the arm-plates. Along each genital 
opening, between it and the arm, and not visible from the outside, 
runs a narrow, sloping piece, the genital plate (seuéwm genitale), 
Its narrow end is turned inward and sometime touches a terminal 
piece, running from the lateral mouth-shield upwards. The out- 
side end of the genital plate is joined with a smaller supplement- 
ary piece, which extends vertically upwards and unites again 
with the radial shields, at a point near the edge of the di 
These parts are never wanting: they are present, even when all 
other portions of the skin-skeleton have disappeared. Ba 
_ the various plates and shields covering the dise are reckon 
first: the mouth-shields (seuta oralia), five in number, ranged in 
a circle about the mouth and placed in the interbrachial spaces, 
just outside the mouth-frames. One of these may bear the 
madreporie body, and is then usually somewhat different from 
its companions in shape. The madreporie body appears as a 
slight depression or elevation on the surface and communicates 
beneath with the “stone-canal.” Along the edge of the madre- 
oric mouth-shield there are sometimes pores.* Secondly: the 
ateral mouth-shields (scutella adoralia), which are just inside 
* See J. Miller: 
Uber die Gattungen der Seeigellarven, 1833, e 38, and 
Le Conte, Proc. Phil. Acad. v, p. 317, 18. ee: 
SECOND SERIES, Vot. XXVIH, No, 82—JULY, 1859. 
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