Echinoderm Larvce with Two Water- Vascular Systems. 329 



employed in previous work on Echinoderm larvae. They were embedded in 

 celloidin, and subsequently in paraffin, by the method which I described in 

 detail when giving an account of the development of Echinus esculentus (12). 



The ISToRMAL Dbvelopmbmt of Echinus miliaeis. 



In a former communication to this Society (12), I have described in detail 

 the development of Echinus esculentus, and Echinus miliaris, the larva of 

 which was first described by me in 1898(11), develops in essentially the 

 same way. Nevertheless, it seems desirable to recapitulate briefly the 

 principal points in my paper on the development of Echinus esculentus, in 

 order that the reader may more easily grasp the results of the experiments to 

 be described in this paper. 



The result of the segmentation of the egg is to form a spherical blastula, 

 one side of which then becomes flattened, and an invagination of the centre 

 of this flattened surface forms the primordial gut or archenteron, and 

 so the blastula is changed into a gastrula. From the apex or blind end of 

 the archenteron a bilobed vesicle is cut off ; this is the rudiment of the 

 secondary body-cavity or coelom ; the wide space which intervenes 

 between the archenteron and the ectoderm, and which contains a few con- 

 nective tissue cells, is the blastocoele or primary body-cavity (fig. 1). 



The larva now grows in length, and becomes obliquely flattened on its 

 anterior surface ; it is as if it were bevelled. This new flattened surface is 

 the oral surface, and iu the midst of it the mouth is formed as the 

 opening of a wide shallow invagination, which is termed the stomodseum. 

 The archenteron having given off the ccelomic rudiment now becomes con- 

 verted into the larval gut. It was originally straight, but it now becomes 

 arched, so that its apex is bent towards the bottom of the stomodaeum, and it 

 becomes divided by constrictions into three portions, viz., oesophagus, 

 stomach, and intestine. The oral surface of the larva becomes sur- 

 rounded by a thickened ridge of ectoderm, which carries very long and 

 powerful cilia, and so the ciliated band, which is the organ of locomotion 

 of the young larva, is formed. 



Soon after, the bottom of the stomodaeum breaks through into the 

 oesophagus, and so the formation of the larval gut is completed. The anus 

 is, of course, the persistent blastopore or opening of the invagination 

 which made the archenteron. The coslomic rudiment becomes divided into 

 right and left ccelomic sacs at the same time, and the ciUated band 

 grows out into four lobes. These lobes are the first of the larval ciliated 

 arms to be formed ; two are situated in front at the sides of the mouth and 

 are termed antero-lateral, and two are placed behind the mouth in front 



