DEVELOPMENT. 



39 ■ 



which, at its anterior end, lies above the note-chord; an 

 anterior dilatation, which is sometimes called the " heart " ; 

 a ventral vessel beneath the gut; and numerous smaller 

 vessels. The blood flows forwards dorsally, backwards 

 ventrally. This system should be contrasted with that of 

 Amphioxus. 



Excretory and reproductive systems. — The excretory 

 system is slightly developed. No nephridia are known, 



Fig. 171. — Development of Balanoglossus. — After Bateson. 



The mesoderm is represented by the broken dark line. 

 In the upper row, from the left — 

 Section of blastula ; beginning of gastrulation, End., endoderm ; 

 section of gastrula, 111., blastopore; Ac, Archenteron ; S.c, 

 segmentation cavity ; closure of blastopore, outgrowth of five 

 coelom pouches (M.). 

 In the lower row, from the left — 

 Longitudinal section, showing the five parts of the body cavity 



(be 1 ., ic%., 6c 3 .) or coelom. 

 Cross-section, C.N.S., central nervous system ; Nch., notochord ; 



b&., body cavity in collar region. 

 Section at a later stage, D.b.v., dorsal blood vessel. 



but from the region of the collar two ciliated funnels open 

 to the exterior, and we have already mentioned the 

 enigmatical proboscis gland. 



The sexes are separate. A number of simple paired 

 genital organs lie dorsally in a series on each side of the 

 body cavity in and behind the region with gill-slits 

 (Fig. 170, It.). They open by minute dorsal pores in the 

 skin, or in the American species by rupture. 



Development. — The eggs must be fertilised outside of 

 the body. Segmentation is complete and approximately 



