xvi ECHINODEKMATA 547 



rings are in active division. Some of the cells constituting the apical 

 field lose their cilia and retreat to the bases of their neighbours, and 

 here form ganglion cells ; in this way an apical plate comparable to 

 the apical plate of the Trochophore larva is formed, from whence a 

 cord of nerve fibres extends down on each side of the ventral surface 

 of the larva. 



Whilst these changes are going on the anterior vesicle of the 

 archenteron becomes constricted into a narrow, elongated dorsal 

 portion and a spherical ventral portion. The former is the rudiment 

 of the anterior coelom, the latter of the hydrocoele. 



During the fourth day the dorsal and ventral horns, which grew 

 back from the anterior vesicle, unite and form the gut, which thus 

 at first has the form of a ring surrounding the narrow middle part of 

 the dumb-bell-shaped posterior vesicle. The narrow part of the dumb- 

 bell becomes a solid cord of cells and then disappears, and the ring- 

 shaped gut becomes a sac by the filling up of the cavity of the ring. 

 The right and left halves of the posterior vesicle become the 

 right and left posterior coeloms respectively. These shift slightly 

 one from another so that the right extends slightly on to the dorsal 

 side of the embryo, the left on the ventral surface. 



The anterior coelom, just after it separates from the hydrocoele, 

 sends out on the right side a small outgrowth. This is probably the 

 rudiment of a right hydrocoele. The anterior coelom becomes 

 completely constricted off from the hydrocoele, and both are divided 

 from the definitive gut. The primary madreporic pore is formed 

 by an outgrowth from the hinder end of the anterior coelom meeting 

 the ectoderm on the left near the ventral line. 



The larval stomodaeum then appears as a thick -walled in- 

 vagination on the ventral surface of the larva between the second and 

 third ciliated rings, which it presses apart. It is therefore evident 

 that these ciliated rings are not strictly comparable with either those 

 of the Citcumaria larva or the pupa of Synapta. The two cords of 

 fibres proceeding from the apical nervous system pass down its sides. 

 Behind the apical field and in the gap of the first ciliated band is a 

 small glandular pit in the ectoderm. This is the fixation pit, by 

 which the larva eventually attaches itself to the substratum. The 

 floor of the stomodaeum does not come into contact with the wall of 

 the gut, so that the larval alimentary canal, which is devoid of an 

 anus since the blastopore became closed, does not acquire a mouth, 

 but it does come into close contact with the hydrocoele which is 

 flattened against it. 



During the fifth day the embryo escapes from the egg-membrane 

 as an oval larva and swims about by means of its ciliated bands. 

 This phase of its development may last from a few hours to four or 

 five days. About this time the calcareous ossicles can be made out, 

 embedded in the mesenchyme. Of these we have two oblique rings 

 of five ossicles encircling the hinder part of the body the hinder 

 ring is known as the oral ring of plates, the front one as the basal 



