No. 3.] GROWTH AND METAMORPHOSIS OF TORNARIA. 42 1 



anterior to the oesophagus. The larva decreases greatly in size 

 during tlie period of metamorphosis. The longitudinal ciliated 

 band becomes more indistinct, and the anterior region of the 

 Tornaria elongates to form the proboscis. The circular ciliated 

 band moves back farther and farther by the lengthening of the 

 body in the region between the lower limb of the longitudinal 

 and the circular band. The ciliated bands of the forming 

 proboscis become very faint, and there is a tendency for the 

 whole surface of the proboscis to become rounded by obliterat- 

 ing the furrows in the circumoral area. The larva moves 

 about over the bottom by means of the circular band of cilia, 

 which do not vibrate now so rapidly as in the younger stages. 



The most important change which takes place during this 

 period is the formation of the dorsal nerve chord, which is 

 intimately connected with the formation of the collar. During 

 the period of metamorphosis there appears a narrow groove 

 running around the embryo parallel to the lower limb of the 

 longitudinal band, and between it and the circular band, nearer 

 to the former. Fig. 39, PI. XXVI., shows this groove at^-, and 

 the level of the lower limb of the longitudinal band is shown 

 by /. Between these the ectoderm is thickened and filled 

 with deep-stained gland cells. This is the beginning of the 

 collar. In the mid-dorsal line the grooves from the sides do 

 not meet, but each turns forward and becomes continuous 

 with the lower limb of the longitudinal band. This is shown 

 in surface view by Fig. 46, PI. XXVII., which shows part of 

 the mid-dorsal surface. At / and /' are seen the parts of the 

 longitudinal ciliated bands of one side, which become continu- 

 ous before reaching the mid-dorsal line. Also on its opposite 

 side the same band is seen. At g is seen the groove on each 

 side which marks the posterior limit of the collar. Before 

 reaching the middle line, each groove turns forward and be- 

 comes continuous with the longitudinal ciliated band of that 

 side. There is thus left in the dorsal mid-line a plate of ecto- 

 derm, which is found to lie at a lower level than the surface of 

 the collar. It is still, however, continuous with the general 

 ectoderm of the surface. After this the two dorsal ends of the 

 collar begin to roll over towards one another, as shown in 

 Fig. 47, and at the same time the plate of ectoderm sinks 

 beneath them and out of sight. The collar groove becomes 



