576 



INVEETEBEATA 



CHAP. 



poe-^ 



canal, formed, for a time, little solid disc-like bodies applied to the 

 sides of the stomach and intestine. Only very gradually did they 

 acquire distinct lumina, and extend dor sally and ventrally so as to 

 encircle the gut and meet their fellows in the mid-dorsal line. Their 

 extension in an antero-posterior direction was very restricted until 

 the very close of larval life. 



When we examine the structure of the full-grown Tornaria larva 

 before metamorphosis sets in, we find that the longitudinal band of 

 cilia, in addition to becoming differentiated into prae-oral and post- 

 oral loops, has been thrown into several secondary folds which we 



may term arms. If we applied 

 the nomenclature which Mor- ' 

 tensen has invented for Echino- 

 derm larva (see p. 464), we 

 should say that the longitudinal 

 band, in addition to giving off 

 a backwardly directed prae- 

 oral loop, gave off also a 

 forwardly directed anal loop. 

 Where the anal loop is given 

 off there is, on each side, a 

 conspicuous postero - lateral 

 arm of the band. From the 

 sides of the prae-oral loop a 

 large prae-oral arm is given 

 off, and on the main portion 

 of the band, just behind this 

 loop and behind the apical 

 plate, is also to be found a 

 backwardly directed antero- 

 dorsal arm; on the left side 

 is the proboscis-pore. 



The ventral ectoderm form- 

 ing the oral field, included 

 between prae-oral and anal 

 loops, is thin and flat. Outside this area it is composed of cubical 

 cells, and it becomes quite thick on the anterior part of the larva 

 around the apical plate, and to a lesser extent around the anus 

 within the area included within the posterior ciliated band. The 

 epithehum hning the dorsal and ventral walls of the oesophagus 

 is ciliated ; the dorsal cilia stop short of the stomach, but the ventral 

 ciliated band is continued into the general ciliation of the walls of 

 the stomach. 



The sides of the oesophagus are produced into several pairs of 

 pockets, these are the rudiments of the future gill pouches {g.f, 

 Fig. 421). It will be particularly noted that in the Tornaria larva 

 the gill region is far in front of the middle or collar body -cavity 

 which embraced the hinder end of the stomach, whereas in the adult. 



Fig. 420. — Illustrating the origiu of the middle 

 and posterior coelomic vesicles in the New 

 England Tornaria. (After Morgan.) 

 A, longitudinal section through the anterior portion 

 of the wall of the stomach, in order to show the origin 

 of the .collar cavity. B, longitudinal section through 

 the posterior portion of the wall of the stomach, and 

 through the wall of the intestine, in order to show 

 the origin of the trunk-cavity. co<i^, rudiment of collar- 

 cavity ; eoe3, rudiment of trunk-cavity ; iTti, intestinal 

 wall ; sL stomach wall. 



