110 GEORGE W. FIELD. 



the enterocoels occurs before the formation of the larval 

 mouth. Asterids form an exception. Still this condition 

 occurs in some Asterids^ e. g. Asterina.^' It would seem as 

 if too much importance has hitherto been attached to this 

 point, since it is subject to so much individual variation. 



After separation from the archenteron the enterocoels in- 

 crease slightly in size and move nearer to the dorsal wall of 

 the larva, now appearing as ovoid vesicles with walls formed 

 of flattened mesenchyme-like cells, which send out branching 

 processes ; these processes uniting with the branches of the 

 mesenchyme-cells, which form an anastomosing network 

 within the segmentation cavity, serve as supports for the 

 enterocoels. 



Formation of the Water-pores and Pore Canals. — Soon 

 after the completion of the larval digestive tract by the fusion 

 of the stomodseal ingrowth with the evaginated portion of the 

 archenteron (fig. 11) begins the formation of the water-pores 

 and the pore canal. On the dorsal wall of the enterocoel a 

 diverticulum is formed. The cells of this diverticulum take 

 on a cubical form ; the cells of the rest of the enterocoel wall 

 retain the flattened branching appearance characteristic of 

 mesenchyme (fig. 16). Above this upward projection of the 

 enterocoel wall there appears a proliferation of the dorsal ec- 

 toderm. This as a solid plug extends downwards, meets, and 

 fuses with the upward projection of the enterocoel; a cavity 

 becomes formed in this ectodermal portion, and through it the 

 cavity of the enterocoel is put in communication with the ex- 

 terior. In this manner a right and a left water-pore and 

 pore canal are formed. The walls of the pore canal are formed 

 of columnar cells, which become ciliated. The pore canal is 

 thus found to be made up of mesodermal and ectodermal ele- 

 ments. These observations are very unlike those described by 

 Bury in his account of the mode of formation of the pore 

 canal and water-pore in another Echinoderm. He says, 

 " Examination of the living animal under a very high power 

 shows that this pore is formed by a single elongated cell, 

 perforated throughout its length and lined with cilia" (5, 



