Notes on the Embryology of Starfishes. 213 



mouth {m), in the deep horseshoe-shaped groove {g) separating 

 the anal and oral plastrons; this groove flares out somewhat 

 towards the edges and gradually grows thinner on the aboral 

 side (Figs. 2, 7, g) where it terminates. 



The water system in the young Brachiolaria (w, Fig. 11) 

 forms the principal part of the body of the Pluteus extending Y- 

 shaped, so as to inclose within its shanks the whole digestive 

 cavity, and opening externally on the aboral side, through the 

 water-pore. In the youngest Tornaria (Figs. 2, 3) the water 

 system consists of a single conical cavity opening outwards 

 through a long tube at b (Figs. 2, 3), sending out two short 

 processes inclosing the oesophagus at its junction with the sto- 

 mach and attached to the oral extremity by a strong muscle (m'), 

 the two short processes of the conical water system are the only 

 trace of the marked Y-shaped cavity which incloses the whole 

 of the oesophagus in the Brachiolaria. The opening (b) of the 

 madreporic canal is situated slightly on one side of the median 

 line (Figs. 2, 7, b). 



The mode of formation of the water system, though I have 

 not traced it, is probably similar to that of the Brachiolaria, as 

 a diverticulum of the digestive cavity; its further increase, 

 however, takes place in a very different manner. Owing to 

 the early development of the oesophagus in the young Brachio- 

 laria, the earlets of the water system become separated and 

 only ultimately join again by extending on each side beyond 

 the mouth,* and thus form the Y-shaped system which then 

 gradually extends and incloses the stomach within its anal 

 branches. In the Tornaria the oesophagus does not take this 

 extraordinary development when compared to the remainder of 

 the digestive cavity ; hence the water system remains united 

 as a single cavity, most probably pushing its way out to the 

 surface and communicating outwardly by means of the water 

 pore {b) in the same manner as in Brachiolaria. 



* See my paper on the Embryology of the Starfish, p. 17. 



