CHAEACTER OF THE DE^^XOP]SIEXT. 55 



FUj. 8), the rudimentary locomotive organs, laid out during the Brachina 

 stage, attain their greatest development, as long, slender arms. The great 

 changes which take place on the anal extremity of the water-tubes on 

 both sides of the stomach, characterize the present stage (the Brachiola- 

 ria stage). These changes upon the surface of the two branches of the 

 water-tube lead to the formation of the future Starfish. But the incipi- 

 ent Starfish is, as it were, a part of the Brachiolaria, or rather the Bra- 

 chiolaria is undergoing local transformations w'hich lead to the formation 

 of a Starfish. They present thus, for a time, the appearance of a double 

 existence, as if a new being were forming in one which had completed 

 its growth. This third period, during which the twofold nature is pre- 

 sex'ved, is the one which constitutes the Brachiolaria stage. In the Bra- 

 chiolaria stage there are several marked periods : the parts which appear 

 at first on the surfaces of the water-tubes have no connection, and stand 

 in such indefinite relation to each other, that they do not seem to tend 

 towards a common result. But in proportion as the young Echinoderm 

 progresses in its development, the relations of the two areas, formed on 

 the surfaces of the two water-tubes, are more apparent ; and we finally 

 reach the last of the strictly larval stages, when the Brachiolaria, with 

 its complicated system of locomotive appendages, becomes secondary to 

 the young Echinoderm and is completely resorbed by it, when the em- 

 brj'O enters into its truly echinoderraoida: condition (PI. VI. Fig. 1), the 

 different stages of which we have already described. 



Examination of the Character of tlie Development. — The mode of develop- 

 ment of Starfishes, thus divided into phases as observed in our Astera- 

 canthion, cannot be called a case of alternate generation, nor is it a 

 metamorphosis in the ordinary sense of the word. It is a mode of 

 development peculiar to Echinoderms, entirely different from that of 

 any other class of Eadiates. It is not an alternate generation, for the 

 Brachiolaria can in no way be called a nurse, as each Brachiolaria pro- 

 duces but one Stai-fish, and the whole of the larva is resorbed by the 

 Starfish, not an appendage being left out. Nor is it strictly a metamor- 

 phosis, as the changes which take place are so gradual that at no time 

 can the line of demarcation be drawn between two stages with any 

 degree of precision, as in Crustacea or Insects, where the casting of an 

 envelope marks distinctly different epochs. There is, however, something 

 m these successive phases of development which reminds ns of the meta- 



