BIPINNAEIA AXD BRACHIOLAEIA. 67 



noderm, which, at first sight, did not seem to differ very materially from 

 what was known of the development of other Radiates, yet Miiller was 

 the first to trace the wonderful changes of the yomig Echinoderms ; his 

 memoirs have been the basis of all subsequent investigations, which are 

 insignificant when compared to the immense amount of labor involved in 

 his i-esearches. Not alone the history of a single animal, but the history 

 of a whole class, is gradually unfolded in his successive memoirs. The 

 very fact that so little has been done in the embryology of Echinoderms 

 since the days of Miiller — for, in fact, with the exception of Krohn 

 and Thomson, 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 re- 

 quires the most practised eye to detect their presence; their habits also 

 are such that we may spend days in watching for them, without obtain- 

 ing 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 Miiller in fol- 

 lowing out the development of so large a number of species, in a field 

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

 must have been taxed to the utmost. 



Bipinnana and BracJiiolaria. — A glance at the figures of Bipinnaria and 

 of Brachiolaria of PI. IX. of MUller's seventh Memoir will show how dif- 

 ferent they are, with few exceptions, from the figures of the same larvae 

 in his former memoirs ; compare PI. VII. of his third Memoir and PI. 

 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 Star- 

 fishes found by him, the very charactei'S which have enabled me to 

 correct his observations. He has seen the two Y-shaped water-tubes ex- 

 tending 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 tentacu- 

 lar pentagon, nor does he perceive the connection of these two pen- 

 tagons wdth the water-tubes. And, finally, if he had kept his Bipinnaria 

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

 develop, and have satisfied himself that Brachiolaria is only an adult 

 state of what he calls Bipinnaria. It must be remembered, however, 

 that the original Bipinnaria of Sars, the Bipinnaria asterigera, has en- 



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