90 



adductor muscle is attached, the other medially directed, having the shape 

 of a thin plate with serrate edges, but otherwise unfenestrated. No muscle 

 is attached to this plate. The posteriorly directed branch from the ventral 

 recurrent rod coalesces with the end plate of the body rod. (In the paper 

 referred to above the supplementary transverse rod was regarded as the 

 true ventral transverse rod; this was the only possible explanation at that 

 time, before the more primitive forms of this larval type were known. 

 The presence of both the supplementary and the true ventral transverse 

 rods in the species a— d leaves no doubt that the interpretation of these 

 skeletal parts in species e (and f) given here is the correct one). The 

 anterolateral rods are quite rudimentary. As will be remembered there is 

 in the species a— d just outside the base of the anterolateral rod a long 

 thorn proceeding from the postoral rod. In the present species there is only 

 a thorn at the corresponding place, but generally a distinct knob is seen on 

 the proximal side of the base of this thorn. Very probably this knob is 

 all that remains of the anterolateral rod, the thorn itself corresponding 

 to the thorn outside the anterolateral rod in the other species. The point 

 of this thorn serves for attachment of the dorsal adductor muscle. If it 

 could be proved that this thorn also in the other species serves for attach- 

 ment of this muscle, the interpretation here suggested would be definitely 

 proved to be correct; unfortunately I have been unable to ascertain the 

 presence of such dorsal adductor muscle in any of the four other species, 

 so that this point must remain unsettled for the present. — The postero- 

 dorsal rods are simple, somewhat irregularly thorny. The posterior trans- 

 verse rod is situated somewhat dorsally, not in the middle line (Fig. 34 A). 

 It is a most elegantly shaped piece of calcareous structure, a gracefully 

 curved bow, the ends of which are slightly widened and fenestrated. 

 In the middle part it bends over ventrally, forming a triangular fenestrated 

 plate, which covers the abductor muscle. At its posterior side a small 

 separate skeletal part is situated resembUng a small young spine (also 

 recaUing the posterior process of the Spatangoid larvae, with which it 

 can, however, scarcely be homologous, the latter being in direct connection 

 with the posterior transverse rod and an outgrowth therefrom, while in 

 the present case it is a separate piece, independent of the posterior trans- 

 verse rod). Whether a dorsal arch is present or not is hardly ascertainable. 

 The upper part of the body contains some irregular branched rods support- 

 ing the processes at the anterior edge, which look hke but are not really 

 rudimentary anterolateral or preoral arms. It is quite possible that one 

 of these rods really represents the dorsal arch (see PI. XIII, Fig. 4 of species 

 f); that it is in a nearly vertical position may not be a serious objection 

 to regarding it as homologous with the dorsal arch of other Echinoid 



