44 EMBRYOLOGY OF THE STARFISH. Part I. 



marked epoch is that during which the larva passes from the cylindrical, or, as we 

 have called it, the Tornaria stage, into a quadrangular, somewhat compressed form ; 

 and the complicated system of locomotive appendages, so greatly developed in the 

 Brachiolaria, is gradually laid out, thus preparing the larva for the last stages of its 

 existence, characterized by the development of the young Echinoderm. This third 

 stage, corresponding to that observed by Van Beneden, may appropriately be called 

 the Brachina stage. During this period, the former independent water-tubes (w, w) 

 of the Tornaria stage (the problematic bodies of Muller) become united, and are 

 gradually transformed into the Y-shaped, elliptical water-system (the Schlauch-System 

 of Muller) ; this present stage (the Brachina stage) is therefore marked by the great 

 modifications of the water-system (PL II. Mgs\ 25-28 ; PI. III. Figs. 2-10). In the 

 . last stage, which we shall call, with Muller, the Brachiolaria stage (PI. III. Fig. 11 ; 

 PI. VI. Figs. 1, 2, 4 ; PL VII. Fig. 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 Brachiolaria stage). 

 These changes upon the surface of the two branches of the water-tube lead to 

 the formation of the future Starfish. But the incipient Starfish is, as it were, a 

 part of the Brachiolaria, or rather the Brachiolaria is undergoing local transforma- 

 tions which 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 two-fold 

 nature is preserved, is the, one which constitutes the Brachiolaria stage. In 

 the Brachiolaria 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 

 embryo enters into its truly echinodermoidal condition (PL VI. Fig. 1), the different 

 stages of which we have already described. 



Examination of the Character of the Development. — The mode of development of Star- 

 fishes, which we have thus divided into phases, as observed in our Asteracanthion, 

 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 Kadiates. It is not an alternate 

 generation, for the Brachiolaria can in no way be called a nurse, as each Brachiolaria 



