14 EMBRYOLOGY OF THE STARFISH. 



appropriately the function they assume of circulating water through the 

 body of the larva. 



The water-tubes (w, w\ at first (PI. 11. Fign. 7, 8, 9, 12, 13, 14) only 

 diverticula from the main digestive cavity {d), become less and less con- 

 nected with it; and by the end of the second day the constriction at the 

 point of attachment has almost entirely separated them from the diges- 

 tive cavity (PI. II. Fif/s. 15, 10, tv, w'). A marked difference is noticed 

 in the rapidity of growth of these two bodies ; the rightrhand one (?//), 

 when the anus is placed in advance, and the mouth downwards, increases 

 more rapidly, extending towards the dorsal side, which it eventually reaches, 

 opening into the surrounding medium by a small aperture (PI. II. F'lrj. 17. h), 

 the water-pore, or, as Miiller has called it, the dorsal pore. A comparison 

 of Fi(ji5. 8 and 18 of PL II. will perhaps render more evident the trans- 

 formation of the diverticula {lo, w) from the digestive cavity into two 

 separate bodies. All we have to do is to swell out the lobed pouches 

 (iv, tv') of Fig. 8, PI. IL, then cut them off, removing them a short distance 

 from the digestive cavity, and we shall have the two independent bodies 

 {tu, iv) of PI. IL Fig. 18, which have little by little been changing their 

 relation to the digestive cavity, as described above. This transformation 

 I have actually observed in every stage of its progress, as it is repre- 

 sented here isolated (PI. II. Figs. 9-16). 



The walls of the oesophagus (o), of the digestive cavity (d), and of the 

 intestine (e), which up to this time are of nearly the same thickness, quite 

 rigid, capable of very limited expansion and contraction (PI. II. Figs. 2, 4, 

 5, 7, isolated, Figs. 10, 11), lose their imiform character with the gradual 

 circumscription of these three regions. The walls now become quite dif- 

 ferent in their appearance, and the more marked the separation between 

 these three organs, the greater the difference in tlie character of the walls 

 which circumscribe them (PI. IL Figs. 17, 19, 21, 23). In proportion as 

 the stomach (d) grows more' spherical, the angle between it and the intes- 

 tine (c) is more acute, apd the intestine [c) becomes a longer and narrower 

 tube, with walls much less thick than those of the stomach [d). The walls 

 of tiie oesophagus (o) are even more flexible ; the conical tube, leading 

 IVoiii the inoutli to the stomach, widening and taking a pistol-shaped form, 

 the Willis Ikivc become so movable, that tiu- opening leading into the 

 stoniiuh can be closod and opened by the greater power of expansion and 

 contraction of this part of the walls (PI. IL Figs. 23, 25). The mouth (;;/), 



