XVI ECHINODEEMATA 495 



perihaemal canals, which are thus formed in a manner similar to 

 that described for Asterias. One of these rudiments arises from the 

 left anterior coelom. 



The main difference between these perihaemal rudiments and the 

 corresponding structures in the Asteroid larva is, that in the 

 Ophiuroid they are thick-walled and their cavities are mere slits 

 (p.h, Fig. 378), whereas in Asterina gibbosa, at any rate, they are 

 thin-walled and open by wide mouths into the coelom. 



On the stomodaeal ectoderm, just over the places where these 

 perihaemal rudiments are situated, five ridges make their appearance 

 which radiate inwards towards the centre of the mouth. The crests 

 of these ridges give off diverging lamellae to right and left, which 

 may be termed epineural flaps (ep, Fig. 378). These lamellae meet 

 those of adjacent ridges^ and thus form roofs over the basal portions 

 of the lobes of the hydrocoele, i.e. those portions which will form the 

 radial canals. In this way is formed the epineural roof which covers 

 in each radial nerve cord of the adult. 



Stage B. — The changes which have just been described go on 

 simultaneously, and constitute what we may call stage A of the 

 metamorphosis. In the next stage, which may be termed stage B, 

 further changes supervene and the absorption of purely larval organs 

 commences. 



All the larval arms, except the two postero- lateral, become 

 reduced in size, the ectoderm covering them retreats to their bases 

 and is devoured by phagocytes, and the spines of the larval skeleton 

 are exposed and broken off. In the meantime the lobes of the 

 hydrocoele each develop two pairs of lateral lobes, the rudiments of 

 the first two pairs of tentacles representing the paired tube feet of 

 Asteroidea {b.t, Fig. 379). 



The perihaemal spaces extend outwards, — i.e. as in Asteroidea, 

 the adjacent sides of two neighbouring perihaemal spaces become 

 apposed to form the two radial perihaemal canals of each arm, 

 which later fuse into one. Branches of these canals extend along 

 the surfaces of the lateral tube feet. 



The outer part of the stomodaeum, which enclosed the primary 

 lobes of the hydrocoele, shallows out and disappears. The adult 

 mouth is formed about the spot where stomodaeum and endodermal 

 oesophagus meet one another ; in a sense it is identical with the larval 

 mouth, because it is, as it were, formed from the deepest recess of the 

 structure, but it cannot be too strongly emphasized that in Ophiuroids, 

 as in Asteroids, the stomodaeum is a temporary structure. 



In the Asteroid the adult mouth is formed to the left of where the 

 larval mouth and stomodaeum were situated. Now, in Op)hiothrix, in 

 this stage, the stomach and intestine are displaced to the right, and 

 this is obviously the same as saying that the mouth moves to the 

 left, because in speaking of the Asteroid the stomach and intestine 

 are taken as fixed points. If we were to regard the outer end of the 

 endodermal oesophagus in the Asteroid, i.e. the inner end of the 



