ON DEYELOP."\rENT ETC. OF FIIORONTS. f)?! 



dorsa] point. Tlie relative position of it with regard to the sep- 

 tum and the tentacles can be most conveniently studied ou sagittal 

 sections of the larval hody. In fig. Go a the adult collar cavity 

 {s.c.c.) is indicated l)y a vertical club-shaped space just inside the 

 the l)ody walls and al)ove the postoral septum (??i(".s'.). Fig. (')ö <J 

 shows a portion of the ventral part of a nearly median sagittal 

 section similar to Fig. 63 a. Here the cavity {s.c.c.) is seen also 

 as a vertical and compara tivel}^ wide space situated inside the 

 body walls and Ix'low the tentacles {p.t.). It can also ])e made 

 out that tlie wall of the cavity is formed of a single layer of 

 mesoblastic cells and its ventral wall is in contact with the somatic 

 Avails of the adult tentacle {s.L), while the ])Osterior wall is super- 

 2X)sed on the anterior side of the se2:)tum (^mcs.). In fig. ~)Si a and 

 oSl) the two cellular circles [s.r.e.) attached to the inner side of 

 the tentacles (/'.) I'epresent a somewhat ol)liquely cut transverse 

 section of the adult collar cavity. The two tentacles belong to 

 the first pair, and the fiirthest dorsal point reached by the cavities 

 is, therefore, at the bases of these tentacles. When the serial 

 sections are traced posteriorly, these cavities gradually extend more 

 and more ventrally along the body walls and at last join with 

 each other in the median ventral line. In fig. 58 c the cavities 

 appear as two narrow s])aces {s.c.c.) appressed against the ectoblast 

 {ech). In fig. 55 a, which is taken from a cross section of an 

 yl-type larva cut nearly parallel with the tentacular row, the 

 cavity {s.c.c.) appears as a long slit-like space intervening l)etween 

 the postoral septum {)nes.) arid the l)ody walls. Here the cavity 

 is seen entirely free of the septum, Ijecause the section passes 

 through that portion of the collar which lies slightly al)Ove the 

 somatic insertion of the septum (compare fig. 63 d). 



In somewhat younger larva? of all types, the adult collar 



