12 EMBRYOLOGY OF THE STARFISH. Part I. 



growth, and which he has called Schlauchsystem, when they appear, in the older 

 lame, as broad tubes running on each side of the oesophagus and stomach. These 

 parts he considered as independent systems, but as they are only different stages of 

 the same thing, as will appear below, they have received here the name which 

 denotes most appropriately the function they assume of circulating water through 

 the body of the larva. 



The water-tubes (w, «/), at first (PL II. Figs. 7, 8, 9, 12, 13, 14) only diverticula 

 from the main digestive cavity (d), become less and less connected with it; and, by 

 the end of the second day, the constriction at the point of attachment, has almost 

 entirely separated them from the digestive cavity (PI. II. Figs. 15, 16, w, w'). A 

 marked difference is noticed in the rapidity of growth of these two bodies ; the right 

 hand one (««/), when the anus is placed in advance, and the mouth downwards, in- 

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

 opening into the surrounding medium by a small aperture (PI. II. Fig. 17, b), the 

 water-pore, or, as Miiller has called it, the dorsal pore. A comparison of Figs. 8 and 

 18 of PL II, will perhaps render more evident the transformation of the diverticula 

 {iv, to') from the digestive cavity into two separate bodies. All we have to do is 

 to swell out the lobed pouches {to, w) of Fig. 8, PL II., then cut them off, remov- 

 ing them a short distance from the digestive cavity, and we shall have the two 

 independent bodies {iv, to') of PL II. 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 represented here isolated 

 (PL II. Figs. 9-16). 



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

 which up to this time are of nearly the same thickness, quite rigid, capable of 

 very limited expansion and contraction (PL II. Figs. 2, 4, 5, 7, isolated Figs. 10, 11), 

 lose their uniform character with the gradual circumscription of these three regions. 

 The walls now become quite different in their appearance, and the more marked 

 the separation between these three organs, the greater the difference in the char- 

 acter of the walls which circumscribe them (PL II. Figs. 17, 19, 21, 23). In 

 proportion as the stomach (d) grows more spherical, the angle between it and the 

 intestine (c) is more acute, and the intestine (c) becomes a longer and narrower tube, 

 with walls much less thick than those of the stomach (d). The walls of the oesopha- 

 gus (o) are even more flexible ; the conical tube, leading from the mouth to the 

 stomach, widening and taking a pistol-shaped form, the walls have become so mov- 

 able, that the opening leading into the stomach can be closed and opened by the 

 greater power of expansion and contraction of this part of the walls (PL II. Figs. 23, 

 25). The mouth (m), as it increases in size, grows triangular, with rounded corners ; 

 the depression in which it is placed, divides the larva into two very distinct regions 



