54 EMBRYOLOGY OF THE STARFISH. 



to be the young of one of the species of Asteracanthion of the northern 

 coast of Europe. The single row of ambulacral pores, the ocular ten- 

 tacle, the arrangement of the pedicellaria^, the size, all confirm the idea 

 of its being only the young. 



Successive Phases of Devehpmeitt of ihc Larvce of Starfishes. — Before apply- 

 ing the information thus far obtained to the solution of more general 

 problems, it may be well to consider what are the normal stages of growth, 

 at different periods, in the history of our Starfish larvfB. During the 

 earlier stages of its existence, the young developed from the egg (PI. I. 

 Figs. 22 - 28) laid by one of our Asteracanthion has no resemblance what- 

 ever to the future Starfish. This first condition we might call the pyri- 

 form, or Scyphistoma stage ; when it is simply a symmetrical radiate 

 animal, reminding us of earlier stages of Polyps and Acalephs. It then 

 assumes the shape of a dumb-bell, becomes slightly one-sided (PI. II. 

 Figs. 2-19), and has, in its most advanced state, no other appendages 

 but the simple crescent^shaped, slightly undulating, vibratile chord (PI. II. 

 Figs. 20-24). The simple, straight digestive cavity is now differentiated 

 into three distinct regions. This second stage we might call the Tornaria 

 stage, from its resemblance to the Echinoderm larvte, called Tornaria by 

 Miiller, in which all the parts of the adult larva are simply hinted at in 

 the most rudimentary form, and during which it is eminently cylindrical. 

 [This Tornaria has been proved by Metschnikoff and myself to be a 

 young Balanoglossus.] Another well-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 compli- 

 cated system of locomotive appendages, so greatly developed in the Brachi- 

 olaria, is gradually laid out, thus preparing the larva for the last stages 

 of its existence, characterized by the development of the young Echino- 

 derm. 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 («•, iv) of the Tornaria stage (the prob- 

 lematic bodies of Miiller) become united, and are gradually transformed 

 into the Y-shaped, elliptical water-system (the Schlauch-System of Miil- 

 ler); this present stage (the Brachina stage) is therefore marked by the 

 great modifications of the water-system (PL II. Figs. 25-28; PI. III. 

 Figs. 2-10). In the last stage, which we shall call, with Miiller, the 

 Brachiolaria stage (Pi. III. Fig. 11; PL VI. Fig^. 1, 2, 4 ; PL VII. 



