ON THE EMBRYOLOGY OF ECHINODERMS. 



3 



is formed, which goes on increasing in length until we have a hollow cylinder (d) 

 extending half the length of the larva, as in figs. 1 and 2, which correspond to fig. () 

 of my paper referred to above. In the profile, fig. 1, we notice the same tendency 

 in the digestive cavity (d) to incline towards the lower side, after the dorsal portion 

 has increased more rapidly, giving the anal part of the larva a bevelled appearance. 

 In a somewhat older stage, fig. 3, the digestive cavity is still longer, and almost touches 

 the lower side. We notice a difference between the Starfish and the Sea-urchin in 

 the time of formation of the alimentary canal, the stomach, and the oesophagus. In 

 the Starfish the mouth is formed before the differentiation of these organs takes place ; 

 while in the Sea-urchin the mouth is not formed until the alimentary canal and the 

 oesophagus, as well as the water-tubes, are quite distinctly defined. (See fig. 8.) What 

 is also peculiar to Echini is the presence of large masses of yolk-cells along the sides 

 of the digestive cavity, indicative of the great changes which take place at the points 

 where these masses of yolk-cells are most numerous. We have observed that the yolk- 

 cells are always present wherever any new organ is developed; in these larvae the 

 appearance of the water-tubes is preceded by an accumulation of yolk-masses at the 

 extremity of the digestive cavity (see fig. 3) ; and the place of the limestone rods, fig. 

 4, /, is first seen filled by clusters of these yolk-masses, in the midst of which the rods 

 are deposited. Rods extending into the arms are characteristic of Echinoids and 

 Ophiurans; we find nothing of the kind in Starfish or Holothurian larva?. 



In the next stage, fig. 4, the original cylindrical digestive cavity has already a de- 

 cided tendency to differentiation, the walls of the stomach (d) and of the oesophagus (o) 

 being of very different thickness ; from o, the pouches which are to become the water- 

 tubes (w w f ) project far beyond the outline of the digestive cavity. The limestone 

 rods (/ /) can faintly be distinguished from the mass of yolk-cells which surround 

 them. The calcareous cells which take such a great prominence in older larvae (see 

 fig. 19) make their appearance as early as the stage of fig. 3 ; they are quite large in 

 the condition represented in fig. 4. The strong- contrast which already exists between 

 the different parts of the digestive cavity is still more apparent in a stage but slightly 

 more advanced, fig. 5. The dorsal portion of the larva has up to this time been grow- 



ing most rapidly, changing the outline of the larva, particularly when seen in profile ; 

 in the subsequent figures, the outline, when seen from above, is also undergoing great 

 changes. The larva assumes a more rectangular shape (fig. 6) when seen either from 

 above or from below. The water-tubes are almost separated from the digestive cavity, 

 which has been divided into three very distinct regions (c, d, o, fig. 8), the limestone 

 rods, simply T-shaped before, fig. 5, are sending off small processes, and the chords 



