158 



The process having been described in details by Joh. Miiller in his Memoir 

 "Die Ophiuridenlarven des Adriatischen Meeres" in the case of the larva, 

 which he names Pluteus bimaculatus, there is no reason for me to give a 

 detailed description of the process here again, the less so since the partly 

 poor state of preservation of the material available would prevent the 

 completeness of the description. But it is an exceedingly interesting fact 

 that we have in this larva a type of development rather different 

 from that found in the Ophiothrix-larYSi, the only larval type the 

 development of which has been studied in detail as yet. A careful study 

 by means of sections of the developmental processes of the present larval 

 type would be very desirable and would be sure to yield most interesting 

 results. — This is another instance serving to show that it is not justi- 

 fiable to conclude from the study of the development of one single form 

 that the whole of the group follows exactly the same type. (Cf. Antedon). 



As stated above the mediterranean Ophiopluteus bimaculatus agrees with 

 the present larva in the type of its development, and there is certainly 

 no doubt that these larvae are very nearly related. They are alike in struc- 

 ture and shape, and only in minor features specific differences can be 

 pointed out. It might be suggested that perhaps also Ophiopluteus affinis 

 (Echinodermenlarven d. Plankton-Expedition, p. 61, Taf. VI, 1 — 3) will 

 prove to belong to this group. Its general appearance is very much like that 

 of 0. serratus, but its oral structure is incompletely known. The skeleton 

 affords the same general structure, the only essential difference being that 

 the posterolateral rod has a longitudinal ridge, dividing it into two parallel 

 rods. But this would hardly be sufficient to disprove its relationship to 

 the serratus-bimaculatus group. — 



No suggestion can be given as to which Ophiurid is the parental form 

 of these larvae. As stated above, under 0. similis (p. 150) the suggestion 

 that 0. bimaculatus might belong to an Amphiura is untenable, since it 

 was found that the larva of Amphiura filiformis is of quite another type 

 (Ophiopluteus mancus). The presence of the larval type both in the Indo- 

 Pacific Ocean and in the Mediterranean and the Atlantic (— it was taken 

 by Mr. Gruelund, VI. 1920, off Cape Blanco and at the Cape Verde 

 Islands — ) will, of course, be a fact of importance for judging of its parental 

 origin, but it is not sufficient for making a definite suggestion as to that 

 problem. 



Ophiopluteus arcifer nova forma. 

 This larval type is distinguished by the very long median processes from 

 the transverse rods, which in their highest development, in species a, when 

 seen in side view, recall the shape of an oldfashioned cross bow (Fig. 85). 



