XVII 



581 



gon 



tips of these imaginations become fused with the collar coelom on 

 either side, and here the collar pores are formed. The first and second 

 pairs of gill pockets also open into these invaginations, which Morgan, 

 following Bateson, compares to 

 the atrial cavity of Amphioxus. 



The genital organs, which 

 in Balanoglossida are very small 

 and numerous and widely 

 scattered, and of which each 

 possesses its own duct, originate 

 as local proliferations of the cells 

 forming the outer walls of the 

 trunk coelom (gon, Fig. 425). 



The thickened band of ecto- 

 derm cells, marking the site of 

 the posterior ciliated band, per- 

 sists for a long time in the 

 Bahama larva ; and it is possible 

 to make out that the trunk of 

 the young Balanoglossid worm 

 is made up, in about equal 

 proportions, of the regions in 

 front of and behind this band, 

 in both of which very great 

 growth in length takes place. 

 Morgan was able to see the 

 young metamorphosed forms 

 burrowing into the mud at the 

 bottom of his culture vessels. 



The Tornaria found by Eitter 

 (1894) on the Pacific coast of 

 North America, agrees with the 

 Bahama form in having secondary 

 processes (tentacles) developed 

 along the course of the prae-oral 

 processes, and also on the main 

 part of the longitudinal band, 

 but these processes are fewer in 

 number and blunter than in the 

 Bahama larva. There is also an 

 additional pair of processes on 

 the horizontal part of the longi- 

 tudinal band, situated near the 

 mid-ventral line which, following Mortensens' notation, we may name 

 post-oral. As in the Bahama larva there is a second transverse 

 band of cilia situated behind the first main one. 



In this Tornaria, in the mid- ventral wall of the oesophagus there 

 is a thick ridge carrying especially long cilia. This ridge is compared 



425. Portions of transverse sections 

 through the trunk region of three young 

 Balanoglossid worms of different ages, in 

 order to illustrate the development of the 

 genital organ in the Bahamas species. 

 (After Morgan.) 



coe*, trunk-cavity ; gon, developing genital organ. 



