4 ON THE EMBRYOLOGY OF ECHINODERMS. 



of vibratile cilia (», t/), which were a simple button (v, fig. 5), are quite stout, project- 

 ing beyond the general outline when seen from above or below (fig. 6, v). It is at this 

 advanced condition only that the oesophagus touches the lower surface, previous to 

 the formation of the mouth, which takes place only when the larva has reached the 

 condition of fig. 9. A view of fig. 6 seen from the anal extremity, fig. 7, shows how 

 far it has lost its cylindrical shape and become wedge-shaped. In fig. 8, which is the 

 same larva seen in profile, the indentation which indicates the position of the mouth 

 (m) has changed somewhat the even outline of the lower surface ; there is a marked 

 bending of the alimentary canal, bringing the anal opening (a) still nearer the lower 

 surface. In fact, since the first formation of an opening in fig. 1 (a w), which is at 



trictly mouth and 



a continued tendency to bend the anal part of 



the digestive cavity towards the oral surface, even while this single opening performs 

 the functions of mouth and anus, during the period which precedes the formation of 

 the mouth, after the alimentary canal, the true stomach, and the oesophagus have 

 been differentiated. This is somewhat different from what we notice in the Starfish 

 embryo, where the mouth is formed before the anus has been much bent from its 

 original position* 



The large accumulations of yolk-masses round the rods r' / cannot fail to be noticed 

 in the stages just described, figs. 6, 7, 8. This contrasts strikingly with the Starfish 

 larvae, in which we find nothing of the kind ; the body of the young embryo is quite 

 remarkable for its great transparency, which only increases with age, while in the 

 Echinus larvae the great accumulations of yolk-masses renders them somewhat opaque 

 even in their early stages, and the increase of calcareous cells in somewhat more ad- 

 vanced forms, as figs. 17, 19, makes it more difficult than in the Starfish to trace 

 accurately the minute changes which the rods and water-tubes undergo. We now 

 come to conditions, figs. 9 and 10, which are sufficiently advanced to enable us by 

 comparing them with still older forms, such as figs. 16, 17, 19, to form a correct idea 

 of the mode of transformation from the shape of figs. 9, 10, to the complicated larva 

 represented in fig. 20. I merely refer to this comparison in a general way, as in the 

 explanation of the different stages it will be carried out more fully, to call attention to 

 the periods which first give us a clew to the development of the different parts, by 

 showing us plainly which portions of the embryo must assume a great prominence, 

 and obtain a more rapid development than others, to pass gradually through the stages 

 which are hereafter figured. In fig. 9 the difference in the rapidity of the development 



* See %. 8, Proc. Am. Acad., Vol. VI., quoted above. 



