576 E. W. MAOBIUDE. 



into tlie oesophagus is formed partly from ectoderm and partly 

 from endoderm cells (fig. 30, Cil. ad.). As in otlier Ecliino- 

 derm larvcC the blastopore forms the anus, whilst the definitive 

 gut becomes divided by constrictions into intestine, stomach, 

 and oesophagus, the last-named of which joins with the 

 stomodajum. On the third day the antero -lateral arms make 

 their appearance (fig. 31, a.L), and these are supported from 

 their first inception by branches given off from the primary 

 skeletal rods which support the postero-lateral arms. The 

 posterior ends of these primary rods undergo a branching just 

 inside the vacuolated posterior end of the larva, which is 

 characteristic of Ophiothrix fragilis. Each rod branches 

 into anterior and posterior spikes, and each of these divides 

 into a dorsal and venti'al branch, so that by the juxtaposition 

 of right and left halves of the skeleton a kind of basket is 

 produced. In none of the figures is the forking into dorsal 

 and ventral spikes shown, but the division into anterior and 

 posterior branches is sliown in many of the figures (c f. figs. 

 6-12). 



When once right and left ca3lomic sacs have been established 

 from their inner walls muscular fibres grow out and surround 

 the oesophagus (fig. 31, muse.). Thus constrictor muscles are 

 provided by means of which swallowing movements can be 

 carried on and the larva becomes able to feed. At the same 

 time the madreporic pore is formed. As shown in the trans- 

 verse sections represented in figs. 35 and 36, an ectodermic 

 ingrowth [m.^).) consisting apparently of two large clear cells, 

 meets a short outgrowth (p.c.) from the left coelomic sac, and 

 by the fusion of the two a tube is established which leads 

 from the left coelomic sac to the exterior. This tube may be 

 termed the pore-canal, whilst its opening may be called the 

 primary madreporic pore. This pore, as in Echinus 

 esculentus, is at first markedly on the left side of the larva, 

 but subsequently shifts so as to reach the mid-dorsal line ; 

 indeed, in Ophiothrix it even passes beyond the mid-dorsal 

 line. The left coelomic sac has a wider lumen than the right, 

 thus foreshadowing its eventual conversion into the so-called 



