ON THE AN ATOM Y OF OPHTOTHKIX VARIABILIS, ETC. 



107 



On some Parts of the Anatomy of Ophiothrix variabilis, Dune, 

 and Ophiocampsis pellicula, Dune, based on materials 

 furnished by the Trustees of the Indian Museum, Calcutta. 

 By Prof. P. Martin Duncan, M.B. Lond., F.E.S., F.L.S. 



[Eead 3rd June, 1886.] 



(Plates X. & XI. figs. 21-27.) 



Contents. — 1. Distinctions between Ophiothrix variabilis and Lyman's 

 type, O. quinquemaculata, Mull. & Trosch. — II. Mouth-frames; Jaw- 

 plates; Teeth and Tooth-papillae; Muscles. — III. Radial Shields ; Genital 

 Plates, Scales, and Muscles. —IV. Structure of the Arm-bones. — V. Special 

 and general Remarks upon the Muscles. — VI. The Arm-bones of Ophio- 

 campsis. — VII. Description of Plates. 



I. Distinctions between Ophiothrix variabilis, Dune, and 

 Lyman s type, O. quinquemaculata, Mull. Sf Trosch. 



The structures of the upper surface of the arms within tbe disk, 

 the genital plates and scales, the mouth-frames and the dental ap- 

 paratus, which are seen after clearing out tbe inside of the disk, are 

 exceedingly interesting in Ophiothrix variabilis (Plate X. fig. 1), 

 and it is evident that they differ somewhat from those of the species 

 O. quinquemaculata. Mull. & Trosch., which w r as chosen by Lyman 

 as his type of Ophiothrix (see Lyman's excellent figures, ' Chal- 

 lenger ' Report, pi. xlii. figs. 5-8). In O. variabilis there are 

 six arm -bones between the conjoined mouth-frames and the 

 nodular heads of the genital plates, whilst in Lyman's type there 

 are only four. In this type the upper parts of the first and second 

 arm-bones are not very wide, but the corresponding parts of the 

 first and second arm-bones of the Mergui form are wide ; more- 

 over, when the arms are straight the genital plates of the new form 

 only reach inwards to the level of the second, or the interval 

 between the second and third arm-bones, and not to the first 

 arm-bone as in Lyman's type. When the arm is bent sideways 

 and fixed so, after becoming dry, the genital plate of the inw r ardly 

 curved side of the arm of course reaches further adorally in 

 O. variabilis. The length of the genital plates of both forms is 

 much the same, but the greater number of arm-bones in O. varia- 

 bilis, placed adorally to the outer and more or less fixed part of 

 the plates, permits of greater sideways-bending movements of the • 

 arms. The shape of the upper parts of the arm-bones is very 

 similar in both species ; perhaps the first plate is the larger in the 



