DEVELOPMENT OF CUCUMARIA ECHINATA. 



the other, although situated on the left dorsal side, is the right 

 enterocœle {reti) and extends posteriori)', and is much smaller than 

 the former. 



Doliolaria. 



The right free end of the rudimentary ring-canal, on reaching the 

 midventral line, bends posteriory (Textfig. E, mvr). In this stage 

 four of the primary tentacles are well distinguishable, lying in the 

 middorsal (/,), ri^ht dorsal (t 2 ), right ventral and left ventral inter- 

 radius (t 4 ) respectively. The fifth tentacle, corresponding to the left 

 dorsal interradius, appears later (Texîfig. F, t 3 ), so that a four-tentacled 

 stage is distinctly present. 



The right enterocœle, rapidly growing in size, now extends on to 

 the right side across the middorsal line, and at last its right margin 

 comes in contact on the right side of the gut with the right margin 

 of the left enterocœle, which has been growing in the meanwhile into 

 the right half of the body across the midventral line (Textfig. E, rèn, 

 left). The line of contact runs obliquely from anterodorsal to postero- 

 ventral. The intervening septum then disappears but leaves for a time 

 an oblique incision at either end of the line. 



The other ends of the two enterocœles stand opposed to each 

 other on the left dorsal side, separated by an oblique partition, which, 

 begins near the middorsal line anteriorly, and runs obliquely backwards 

 and to the left. This partition indicates the position of the future 

 mesentery, its course roughly coinciding with that seen in adult 

 Cucumaria (Textfig. G, and Pl. V, fig. 4, mes). 



At about this stage appear three, very rarely four, ciliary bands 

 encircling the posterior half of the body at nearly equal intervals (PI. 

 V, figs. 3 and 4, cb x _^). The preoral lobe and the posterior end of the 

 body are uniformly covered with weaker cilia. The embryo swims 

 about immediately below the surface of the water, usually with its 

 long axis in a vertical position, and rotating around it. 



