228 



dulating rod of Ophiopl. iindultilus. In some forms the posterolateral rod is 

 canaliculate to a greater or lesser extent (0. costatus, opulenlus), in others 

 it is composed of two closely apposed, parallel rods (0. af finis), and finally 

 it reaches its highest specialization in the larva of Ophiura te.i'turata, in 

 which it is fenestrated another feature unique among Ophiurid-larvse. 

 This character also recalls the Echinoid-larvae in which fenestrated rods 

 are of general occurrence; but, as was pointed out already by Metschni- 

 koff 1 ) (comp. also "Echinodermen-Larven d. Plankton-Exp." p. 53) the 

 structure of the fenestrated rods of the 0. texturaia-larva is markedly dif- 

 ferent from that found in the Echinoid-larvse, in accordance with the fact 

 that this character has been acquired separately in the two types and is 

 a highly specialized feature in the Ophiurid-larvse, while in the Echinoid- 

 larvae it is, evidently, a primitive character. 



Some Ophiurid-larvae are more or less rudimentary, showing various 

 degrees of reduction from the typical Pluteus-shape to a mere worm-shape 

 which has no longer any likeness whatever to the Pluteus-form. The first 

 stage of reduction may perhaps be represented by the larva of Amphiura 

 /iliformis, in which the posterodorsal arms have disappeared, the larva 

 otherwise being typical in shape. A somewhat more advanced stage of 

 reduction is probably represented by the larva of Ophiura a f finis or thai 

 figured by Joh. M tiller on Taf. VII, Eig. 5 of his VI. Memoir on the 

 development of Echinoderms. Unfortunately, only the metamorphosis- 

 stage is known, however, so that we do not know how far the reduction 

 has gone; in any case the posterolateral arms are completely developed. 

 A much more reduced stage is represented by the larva figured by Metsch- 

 nikoff (Op. cit. Taf. XII. B. Fig. 45) (Ophiopluteus Meischnikoffi) and 

 a still further reduced stage by the larva figured by Claparede 2 ) (Ophiopl. 

 Claparedei). In these two larva?, which I have reproduced in figure 102, 

 the Pluteus-shape is, especially in the latter, hardly recognizable any more; 

 the rudimentary skeleton of 0. Metschnikoffi still distinctly recalls the 

 typical Ophiopluteus-skeleton, while in 0. Claparedei it is reduced to a 

 pair of simple rods. In the larva represented in fig. 102, C, which was dis- 

 covered by Krohn (Ophiopluteus elongatus) the Pluteus-shape is no longer 

 recognizable, but traces of the larval skeleton are stated by Krohn :i ) lo be 

 found. Through these forms we reach to the larvae of Ophioderma brevispina, 



1 ) E. Met schnikoff. Studien liber die Entwickelung d. Echinodermen u. Nemertineii 

 p. 29. 



2 ) E. Claparede. Beobachtungen iiber Anatomic u. Entwickelungsgeschichte wirbel- 

 loser Tiere. 1863. Taf. I Figs. 1112. 



3 ) "Im Embryo liess sich soweilen ausser kleineren Kalkablagerungen .... deutlich ein 

 gerader, starker, bis in die beiden Enden reichender Kalkstab unterscheiden." A. Krohn. 

 Uber einen neuen Entwicklungsmodus d. Uphiuren. Mull. Archiv 1857. p. 373. 



