BALANOGLOSSUS 



495 



end, forms a heart lying above the notochord, and below 

 a closed contractile dilatation, sometimes called the " peri- 

 cardium." Beside the latter there is a paired " proboscis 

 gland," formed from the coelomic epithelium. There is a 

 ventral vessel beneath the gut ; and numerous smaller 

 vessels. The almost colourless blood flows forwards 

 dorsally, backwards ventrally. This system should be 

 contrasted with that of Amphioxus. 



Excretory and reproductive systems. — No nephridia 

 are 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. 

 280, R.). They open by minute dorsal pores. 



Development. — The eggs are fertilised outside of the 

 body. Segmentation is complete and approximately equal ; 

 a blastula results ; this is invaginated in the normal 

 fashion, and becomes a gastrula. 



The development may be direct without a larval stage, 

 as in Balanoglossus {Dolichoglossus) kowalevskii, or indirect 

 with a Tornaria larva, as in Balanoglossus himiniensis . 



In the direct development 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 move about at the bottom in shallow 

 water. It elongates and becomes more worm-like ; there is an anterior 

 tuft and a posterior ring of cilia ; the primitive gut forms five coelomic 

 pouches ; a mouth and an anus are perforated ; there seem to be no 

 fore-gut nor hind-gut invaginations. Two gill-slits appear ; the regions 

 of the body are defined at a very early stage. 



In the indirect development there is a Tornaria larva, at first bell- 

 shaped. A ventral mouth opens into the curved gut, which is furnished 

 with a posterior terminal anus. A " dorsal pore " leads into a thin- 

 walJed sac which becomes the proboscis cavity of the adult. A pair 

 of coelomic cavities develop from the gut wall, followed by a second 

 pair. Gill pouches arise from the sides of the fore-gut. There are 

 characteristic external ciliated bands, something like those of an 

 Echinoderm larva, for which the Tornaria was originally taken, till 

 Metchnikoff recognised its true nature in 1870. There is also an 

 apical sensory plate (like that of many Annelid trochospheres) with two 

 eye-spots. 



The Tornaria is a pelagic form. After a period of free pelagic life 



