586 E. W. MACBRIDE. 



into V-shaped spaces, one limb of each V extending up each 

 arm adjacent to tlic cavity beneath the lobe of the hydrocrele. 

 From the wall of this extension of the cavity are derived the 

 motor ganglion cells of the ccelomic nervous system (fig. 50, 

 n'e'), whilst the cells of the ectodermic nervous system 

 (fig. 50, n. c.) nre derived from the ectoderm covering the 

 tentacles themselves. The first nerve-fibres are seen in fig. 

 50 as fine wavy lines connecting the two sets of nuclei. 

 Other outgrowths from the wall of this cavity, consisting of 

 larger cells with sparser nuclei, are in all probability to be 

 regarded as the rudiments of the ventral intervertebral 

 muscles, though in the latest stages examined by me histo- 

 logical differentiation had not progressed far enough to be 

 absolutely decisive on this point. Similarly, cellular out- 

 growths from the dorsal coelomic canal (fig. 48, muse.) are 

 in all probability the rudiments of the dorsal intervertebral 

 muscles. 



The intestine is reduced to a small solid vestige consisting 

 of vacuolated cells. The dorsal thin-walled portion of the 

 stomodgeum has completely disappeared, and the whole 

 O3sophagus is now surrounded by a thick wall derived from 

 the adoral ciliated band, chiefly from its left half. In 

 Asteroidea and Echinoidea a new adult mouth is formed by a 

 meeting of ectoderm and endoderm on the left side of the 

 larva; in Holothuroidea, where the larval mouth persists, it 

 moves (as Bury has shown [6] ) at the decisive moment of 

 metamorphosis to the left side; the outer thin-walled portion 

 of the oesophagus becomes evaginated at a later period to 

 form the buccal membrane or peristome, whilst the inner 

 tliick-walled portion persists as the oesophagus. Ophiothrix 

 fragilis resembles in this respect the larva of Holothuroidea 

 in that the permanent oesophagus is formed from the inner 

 portion of the larval one, whilst the outer portion of the 

 larval oesophagus by the shrinkage of the forehead is, so to 

 speak, uncovered, and forms the peristomial membrane of the 

 adult. 



The right hydrococle has, during this stage, a very variable 



