THE DEVELOPMENT OF OPRIOTHRIX FRAGILIS. 559 



of Sciences a paper entitled ^' Uber die Larven nncl die Meta- 

 moi"pliosen der Ophinren iind Seeigel '^ (25), in which he 

 describes the metamorphosis of Plutens paradoxus and 

 shows that it is the Larva of an Ophiurid. A somewhat 

 similar organism also referred to the genus Plutens is 

 shown in this paper to be the larva of an Echinoid, and 

 Miiller adopted the regrettable custom of using the same 

 word "Pluteus" to designate these two quite different larva); 

 this custom has persisted until quite recently and has been 

 the source of much confusion. In the case of Pluteus para- 

 doxus the change into an Ophiurid is traced step by step, 

 nnd Miiller figures without describing it a specimen which has 

 a five-lobed i-osette — the form assumed by the rudiment of 

 the water-vnscular system — on the right as well as on the 

 left side of the oesophagus. In this figure also the metameric 

 division of the coelom is beautifully shown, and, indeed, 

 Midler's drawing could have been used to illustrate this 

 paper. Of course, Miiller was unable to interpret his results 

 fully, and since the coelomic rudiments have extremely narrow 

 lumina it is not surprising that he did not recognise them as 

 the forerunners of the body-cavities of the adult. A second 

 form of Ophiurid pluteus with more slender arms is described 

 in the same paper, and in a later paper (27) read before the 

 Academy in 1851, Miiller describes two further forms of 

 Ophiurid larva) which he discovered during a visit to Trieste, 

 one of which he named Pluteus bimacnlatus from the 

 brown pigment spots with which it was ornamented, the other 

 he recognised as the subject of the present paper — the larva of 

 Ophiothrix f ragilis. In Pluteus bimaculatus Miiller 

 discovered the larval anus of Ophiuroidea, thus showing that 

 the absence of the anus in this class of Echinodermata is a 

 secondary, not a primary affair. The hydrococle or rudiment 

 of the water-vascular system is described as the palmate 

 organ, since it possesses five lobes like the five fingers of the 

 human hand. Miiller saw that it was formed late in larval 

 life, and was at first round in shape. In its later stages it 

 extends ventral to the ossophagus, and each lobe becomes 



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