58 C. F. Liitken on Ophiurans. 
the mouth-shields and vary considerably in shape, position and 
size. They are arranged in pairs, a pair to each mouth-shield. 
Thirdly: on the back of the disc, and placed over the base of 
scales, of a great variety of shapes and sizes, but usually small 
and rounded. mong these scales may be pointed out two 
even to ten. The teeth en are plates, arranged in a vertical 
t 
rep] 
(compare Ophiocoma and Ophiothrix). It is in Asterophyton that 
the perfect homology of these variable organs is distinctly shown; 
in this genus all the chewing apparatus takes on the form of 
sharp spines. Along the underside of the arm runs a double 
row of pores, from which the tentacles protrude, and, on the 
inner side of each pore, one, two, or even four scales or papille 
(papillae ambulacrales) are placed, which serve to cover the ten- 
tacle when it is drawn in. They may, however, be entirely 
wanting (in Ophiomyxa and Ophiothrix). When there are more 
than one on the basal pores of the arm, they decrease in num 
towards the tip. At the outer end of each mouth-slit are two 
tentacles (pedes orales) which are the last pair at the base of the 
arm. These, according to Forbes, are used to remove the undi- 
* For further remarks on the Ophiuran Skeleton, see J. Miller: Uber die Ophiv- 
icigk it iteteches Moun: 1861, p-1,ani Ubec den Bok Sr Bid ond 
men, 1858, p. 51 and 76. 
