Chap. IV. BIPINNARIA AND BRACHIOLARIA. 55 



subsequent investigations, which are insignificant when compared to the immense 

 amount of labor involved in his researches. It is not simply the history of a 

 single animal of a class; it is the history of a whole class, which is gradually 

 unfolded in his successive memoirs. The very fact that so little has been done 

 in the embr}'ology of Echinoderms since the days of Muller — for, in fact, with the 

 exception of Krohn and Thompson, no one has followed these transformations — is a 

 sufficient proof of the great difficulty attending investigations of this kind. It must 

 also be remembered that these animals are so small that it requires the most 

 practised eye to detect their presence ; their habits also are such that we may 

 spend days in watching for them, without obtaining a single specimen, and again 

 be overwhelmed with such an amount of material as to be at a loss where to 

 begin. This can but heighten our admiration of the untiring zeal and perseverance 

 of Muller in following out the development of such a large number of species, in 

 a field where everything was unknown, and where his powers as an observer 

 must have been taxed to the utmost. 



Bipinnaria and Brachiolaria. — A glance at the figures of Bipinnaria and of Brachi- 

 olaria of PL IX. of Midler's seventh Memoir will show how different they are, with 

 few exceptions, from the figures of the same larvae in his former memoirs ; compare 

 PI. VII. of his third Memoir and PL II. of his second Memoir. From the figures and 

 explanations given by the author, it is evident that he had observed, in the last larvae 

 of Starfishes which he found, the very characters which have enabled me to correct his 

 observations. He has seen the two Y-shaped water-tubes extending the whole length 

 of the Bipinnaria. He has seen, also, that the pentagon of the future back of the 

 Starfish was open in its younger stages, though he did not succeed in tracing the 

 position of the tentacular pentagon, nor does he perceive the connection of these two 

 pentagons with the water-tubes. And, finally, if he had kept his Bipinnaria alive 

 but a short time longer, he would have seen brachiolar appendages develope, and 

 have satisfied himself that Brachiolaria is only an adult state of what he calls Bipin- 

 naria. It must be remembered, however, that the original Bipinnaria of Sars, the 

 Bipinnaria asterigera, has entirely different characters from the Bipinnaria of Muller. 

 Judging from the development of our Starfish, it seems to me that Midler's Bipin- 

 naria von Helsingor, second Memoir (PL I. Figs. 1-7), is probably nothing but a 

 younger stage of his Brachiolaria von Helsingor (PL II. Figs. 4, 5; and PL III.). Van 

 Beneden's Brachina, in its turn, is a still younger stage of the same thing, or of 

 an allied species. A comparison of the above figures of Muller, and of the figures 

 of PL III. of this Memoir, will leave no doubt on this subject. For the same reasons 

 the Brachiolaria of Marseilles is probably only the adult of a Bipinnaria, closely 

 resembling that of Marseilles (second Memoir, PL I. Figs. 8, 9), if it is not the same 

 species. In the Brachiolaria figured on Plates II. and IH. of the second Memoir of 



