438 



SUB-PHYLUM HEMICHORDA. 



known, but from the region of the collar two ciliated 

 funnels open to the exterior, and the enigmatical proboscis 

 gland is possibly excretory. 



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. 235, 7?.). They open by minute dorsal pores. 



Development. The eggs are fertilised outside of the 

 body. Segmentation is complete and approximately equal ; 



FIG. 236. Direct development of Dolichoglossus. 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, /., blastopore ; Ac., Archenteron ; S.c., 

 segmentation cavity ; closure of blastopore, outgrowth of five 

 coelom pouches (.#/.). 

 In the lower row, from the left 

 Longitudinal section, showing the five parts of the body cavity 



(be. 1 , bc? t &r.3) or coelom. 

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



A:. 2 , body cavity in collar region. 

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



a blastosphere results ; this is invaginated in the normal 

 fashion, and becomes a gastrula. 



The development may be direct without a larval stage, 

 as in Dolichoglossus kowalevskii^ or indirect with a Tornaria 

 larva, as in Balanoglossus biminiensis. 



In the direct development the blastopore of the gastrula narrows and 

 closes; the external surface of the gastrula becomes ciliated; the 



