12 EMBRYOLOGY OF THE STARFISH. 



diverticula (w, zv), from the digestive cavity. During this time the main 

 digestive cavity has entirely lost its cylindrical form ; it has become nar- 

 rowed at the extremities and bulging in the centre (PI. 11. Fi(/. 8, and 

 isolated, Ft'ff. 9). When seen in profile, and comparing it with earlier 

 stages (PI. II. Fi^s. 2, 4, 5, 7, isolated, Fi(/. 10, a), it is at once noticed 

 that the opening at one end, the present mouth of the larva, has little 

 by little changed from a position at one extremity of the embryo (PI. I. 

 Fiffs. 27, 28, a) to a slightly eccentric one (PI. II. Fif/s. 4, 5, 7). While 

 the present mouth is changing its position from a terminal to an eccentric 

 one, and while the digestive cavity has been expanding at the bottom 

 into a large reservoir, its closed end is bending more and more towards 

 one side (PI. II. Fif/s. 2, 4), until it finally touches the outer wall of the 

 embryo at m (PI. II. Fi(/. 5). At this point of junction an opening is 

 formed, leading into the bottom of the digestive cavity (PI. II. Fi^. 7); 

 this second opening (m) is now the true mouth, and performs hereafter 

 all the functions of a mouth, while the first-formed opening of the young 

 embryo {a, PL II. Fif/s. 2, 4, 5, 7) is restricted in its functions, and per- 

 forms hereafter only those of an anus ; although in the early stages 

 (PI. I. Fif/s. 25, 26, 27, 28; PI. II. Fi^s. 2, 4, 5, 6) it had performed the 

 functions of a mouth. We have thus an apparent anomaly in the fact 

 that the first opening becomes the anus, while the true mouth is only 

 formed afterwards; but this difficulty is readily explained if we compare 

 the functions of this first-formed opening, the so-called mouth, with what 

 we find among Polyps, where one and the same opening performs the 

 double functions of mouth and anus throughout life. 



The diverticula {fv, it/, PI. II. Fiffs. 7, 10) do not extend, as would seem 

 when seen from above (PI. II. Fi//. 8), at right angles from the main 

 cavity, but trend obliquely upwards, as seen in profile (PI. II. Fij. 7), 

 towards the other extremity of the embryo, as in Fi</s. 7, 10, PI. 11. The 

 outer wall, whicli had formed a connection with the closed extremity of 

 the digestive cavity, on the lower side, has been drawn out in the shape 

 of a slender cone {n, PI. H. Fiffs. 7, 10, 11, 14, 17), and becomes the 

 oesophagus, which leads to an opening (;«, the mouth), connecting the 

 ventral side with the digestive cavity. 



Nomenclalurc. — It will materially assist in the explanation of the sub- 

 sequent changes of form, and obviate a great deal of circumlocution, if we 

 at once cjill tlu^ different organs by tlu'ir true names. The original open- 



