BALANOGLOSSUS. 



419 



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. 206, R.}. They open by minute dorsal pores. 



Development. The eggs are fertilised outside of 

 the body. Segmentation is complete and approximately 

 equal ; a blastosphere results ; this is invaginated in the 

 normal fashion, and becomes a gastrula. 



FlG. 207. Development of Balanoglosstts. 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, bl., blastopore ; Ac., Archenteron ; S.c., 

 segmentation cavity ; closure of blastopore, outgrowth of five 

 coelom pouches (J\f.). 

 In the lower row, from the left- 

 Longitudinal section, showing the five parts of the body cavity 



{be 1 ., be-., bc.) or coelom. 

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



bc-.t body cavity in collar region. 

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



The American species (B. kowalevskii) has a simpler 

 development than the others, for it is without a remarkable 

 larval form (Tornaria) which occurs in them. We shall take 

 the simpler case first, though it is probably less primitive. 



The blastopore of the gastrula narrows and closes; 

 the external surface of the gastrula becomes ciliated ; 

 the endoderm lies as an independent closed sac within 

 the ectoderm. Meanwhile the embryo has become or is 

 becoming free from the thin egg envelope, and begins to 



