426 



ZOOLOGY 



species which is in the main described below is B. howalevshii, 

 which, together with B. Irooksii, is found on the east coast of 

 the United States. 



BcdaTwglossus differs from other members of the phylum 

 with which it is grouped in the fact that its body is covered 

 externally with cUia, and in this respect it resembles a Nemer- 

 tine. The ciliated epidermis contains a considerable number 

 of unicellular mucous glands, which secrete a very copious 

 mucus. This secretion serves to line the tubes in the sand 

 and mud in which the animal lives. In one species the 

 mucus hardens into a definite tube, in the walls of which 

 grains of sand are embedded. The body of BalaTwglosms has 

 usually a very characteristic odour. 



The body-cavity or coelom consists of a single unpaired 

 portion occupying the proboscis, a paired portion in the collar, 

 and a paired portion in the trunk. These are derived from 

 five coelomic pouches of the archenteron of the embryo. The 

 original cavities of these are, however, much obliterated by the 

 growth of stellate connective tissue cells (Fig. 245). The spaces 



Fio. 245. — Section, partly diagrammatic, through 

 the respiratory part of the trunk of Balano- 

 glossus. The whole course of a gill - slit 

 could not he seen in a transverse section, as 

 the slits run oWiquely. 



1. Dorsal portion of alimentary canal. 



2. Ventral portion of alimentary canal. 



3. Dorsal nerve cord, in the skin. 



4. Dorsal blood-vessel. 



5. Ovary. 



6. Gill-slit. 



10. 

 11, 



Nerve plexus at the base of the skin. 

 Longitudinal muscle fibres. 

 Connective tissue cells filling up the 



coelom. 

 Ventral blood-vessel. 

 Ventral nerve in the skin. 



between the cells communicate with the exterior in the pro- 

 boscis by a ciliated proboscis pore, — in one species this is 

 paired, — and the right and left collar cavities open to the 

 exterior by a right and left ciliated collar pore. These latter 



