MUSEUM OF COMPARATIVE ZOOLOGY. 257 



To return now to the chewing apparatus, each angle of the mouth has a 

 supporting skeleton (Fig. C) in the form of a V, at whose apex is the jaw- 

 plate (e). As has already been said, each side of this V is composed (wholly 

 or in part) of the halves of one or more arm-bones greatly modified, and 

 called collectively mouth-frames (PL VII., Fig. 5,/). It has generally been 

 assumed that there was only one modified arm-bone in each half of a 

 mouth-frame, but plainly there must be two, because there are two tentacle- 

 sockets (r, r'), in which are lodged the so-called mouth-tentacles ; and in 

 no Ophiuran or Astrophyton is there ever more than one tentacle, on 

 each side, to every joint or arm-bone. When the mouth-frames are care- 

 fully examined, especially if boiled in potash, there is seen to be a line 

 or suture between the wide outer part (/) and the narrow inner point (c). 

 The suture runs nearly vertically through, or a little outside, the hollow 

 for the nerve-ring ('0. and, in some genera, as Ophioglypha, this inner 

 point (r), called the jaw, is easily detached from the outer portion (/), 

 which is more properly the mouth-frame.* This jaw has no tentacle, and 

 is regarded by Midler as an interambulacral piece, which is soldered with 

 its fellow from the side ; and on the angle or point thus made is fixed 

 the jaw-plate (c), which belongs to the skin formation, and which in turn 

 supports the teeth (<l"). It is in Ophiolhrix that the homology of mouth- 

 frames with the innermost arm-bone may most clearly be seen. In Fig. C, 

 which is a diagram of the innermost arm-bone and of the mouth-frames seen 

 from above, it is evident that the former is split nearly to its outer edge, and 

 that its halves arc turned sideways to meet their fellows from the next arm. 

 The angles 7, 8, 9, correspond in the two pieces. This upper portion of the 

 mouth-frame must be considered the first arm-bone having its own tentacle- 

 socket (Fl. VII., Fig. 13,V). The second arm-bone must be placed directly 

 below the first, and so intimately soldered with it as to form one ; it is pro- 

 vided with its tentacle (?•), which is the second mouth-tentacle. On this 

 view, each side of a mouth-frame would consist of three pieces, to wit, the 

 first and second arm-bones and an interambulacral piece. All, however, is a 

 theory, based on the position of the tentacles, and needs demonstration from 

 embryology. 



The skin formation remains to be considered. It is usual to make two 

 distinct divisions, namely, the skin proper, which includes the arm-plates and 

 the plating or scaling of the disk; and the skin appendages, which arc spines, 

 grains, and stumps. Great weight is therefore given, in classification, to 

 these parts ; but, morphologically, they are all the same, — a fact which may be 



* See J. Miiller iiber den Bau der Echinodermen, 1854, Plate VII., Pig. 6,/. 

 This paper, with that of Gaudrv, Pieces Solides ehez les Stellerides, Annales des 

 Scion. Nat., 1S51, p. 339, are the must important for the subject. 



vor.. in. 17 



