{28) 



The mesoderm cells v;hici- in "^it,. -1 are seen applied to 

 tlie veiitral ectoderm of ti: o larva tiavc iiO .; increased consider- 

 ably in number ana have become arraiiijea at quite definite in- 

 tervals. i'Fir.'^A) If tiie ventral surface o ^" trie larva is ex- 

 ajrdned, whilo the larva is alive, it will bo seen t^at the^o 

 cells have become simple muscle cells made up o f two rather del- 

 icate fibers whic-; extend from a large nucleus situatea near 

 the mid-ventral line. These fibers run parallel to one aiioth- 

 er around the wall of the larva. (Fig. 25) 



The whole body cavity back of the mesentery between the 

 cavities of the collar and lobe represents the larval collar 

 cavity of the A.ctinot ro cha and althou£;h its somatic walls are 

 not lined by a perfectly continuous mesodermal epitr.elium yet 

 there are indications that such a lining;; is bein^ formed. The 

 ventral and lateral v/alls o "f* tne stomach, however, are perfectly 

 free from any epithelial covering. In fact, in all the Ac- 

 tinotrochae that I have exairancd, I nave uot been able tc find 

 a mesodermal epithelium covering the ventral and lateral walls 

 o-f the stomach in the collar region. I have never see-,: any 

 sign o:^' mesodermal sac -like formation sucii as occurs in the 

 preoral lobe. 



Roule (20 page 11£) iias described in a conside --ably older 

 larva th.an the one v.-itn ;viiich we are dealing certain mesodermal 



