///. ENTER0P2^'EUSTA. 



513 



Fio, 5b2.—Baln}inr/lossus Jcoivalew^kii* (From Korschelt-Heider, after A. At?assiz.) 

 rf/>, dorsal blood-vessel; e, proboscis; y, sexual res^ion; k, gill reKiiin; ki\ eollai-; 

 vb, ventral blood-vessel. 



The systematic position of tlio Enteropneusta is not settled beyond a 

 doubt. In the possession of gill slits and in the formation of the dorsal 

 nervous system it closely resembles the 

 other chordates, and the resemblance 

 is strengthened by similarities in de- 

 tails of structure of the gills. The 

 advocates of this view recognize the 

 notochord in a l)luid tube, sur- 

 rounded by tough membrane and 

 thickened beneath, which extends 

 from the pharynx into the proboscis. 

 Embryology throws but little light on 

 the problem. Some species have a 

 direct development (tig. 5.53, B, C), 

 while others have a larva (Tornaria, 

 tig. 553, A) which so resembles the 

 larvte of certain echinoderms that 

 it was long held to belong to that 

 phylum. The chief resemblances are in the relations of the ciliated 

 bands to the alimentary tract and in the presence of the proboscis cavity. 



Fig. ,55.3.— .4, Tornaria larva of Ealrnui- 

 (ili'^sits. (Alter MorKan.) «, apical 

 plate ; ar, preoral part of ciliated 

 band ; hc^, hc'^, hc^, ofelomic pouches ; 

 w, moiUh ; p, postoral j)art of ciliated 

 band B, C\ two statues of Bahinn- 

 ifln^su.'i with direct development. i\i- 

 ter Bateson.) a, anus ; he, branchial 

 clefts ; c, collar ; ric, digestive part of 

 alimentary canal: in, intestine; J'c, 

 ' notochord '; ;», proboscis. 



