274 



Prof. E. W. MacBride. 



wedges, which alternate with the five radial canals which have been 

 developed from the hydrocoele (fig. 7). These outgrowths soon lose 



hy. 



inv. 



jTiQ.. 7. Diagrammatic Section of the developing Oral Disc of the future Echinus 

 in a Larva 26 days old. 



Names as before. In addition, am., amniotic cavity ; inv., invaginations of coelomic 

 epithelium, which are the rudiments of the periheemal spaces and of the " lantern 

 ccelom." 



their connection with the ccelom, and become converted into closed 

 sacs. From each of them a narrow outgrowth sprouts out, which 

 insinuates itself between the water-tube and the ectoderm of the base 

 of the tentacle. In this way the radial perihsemal canals are formed. 

 The main body of the sac, however, forms a tooth-sac (fig. 8). From 

 its floor an upstanding projection is formed, which is the root of the 

 tooth, whilst from its sides the alveoli, which constitute the jaw, are 

 formed. Calcification, however, proceeds in these parts only to a very 

 slight extent till after the metamorphosis. 



By the time the larva has attained an age of 32 33 days, the 

 primary tube-feet have become long, arid have acquired their suckers, 

 and, in addition, a curious little sensory prominence, consisting of 

 elongated ectoderm cells situated in the centre of the sucker ; the 

 three pedicellarise mentioned above are fully developed, and the four 

 primary ciliated epaulettes have grown so as to almost meet one 

 another, and thus form a complete circle. In addition, two secondary 

 ciliated epaulettes have been formed in a more posterior position (see 

 fig. 5) ; these extra epaulettes are characteristic of Echinus esculentus as 

 opposed to Echinus miliaris. 



The larva continues to grow up to 36 38 days after fecundation. 

 By that time several important changes have taken place. The hydro- 



