Introduction to Animal Morphology. 209 



early become consfricted into an acorn-like anterior 

 part,- becoming the proboscis, and an oval body. 

 There are in the adult four large vascular trunks, 

 dorsal, ventral, and two lateral ; the first carries 

 blood from behind forwards, and divides at the gills 

 into four branches, of which one enters the respiratory 

 organ. There is one genus, Balanoglossus, whose 

 three species inhabit the Adriatic and the Indian 

 Ocean. The larva has been often confounded with 

 the pseudembryos of Echinodermata, and is named 

 Tornaria. 



CHAPTER XXXI. 



SUB-PROVINCE 2. SCHIZOCCELA. 



CLASS ii. CH^ETOPODA (Van Bcncdcrf.lhz single 

 class of this sub-province includes the Deuterostome 

 worms whose cceloma is formed by a splitting of the 

 germinal layers. They are cylindrical or flattened, 

 of several (2-400) metameres or zonites.* The first 

 (prostomium), second (peristomium), and last are 

 onomous, the others homonomous.t Some secrete 

 a cuticular or calcareous tube, or agglutinate sand 

 into a case. Each zonite bears bristles, rarely scat- 

 tered over tlv! surface, usually in diverging bundles, 

 often around a o-ntral acicitla or seated on a torus, and 



' urkincllaarc intermediate between the simple and 

 truly join*' 



r some central zonitcs hctcronomous from the development of the 

 sex-or. 



P 



