150 HTATT, 



blood in a healthy state. It would be exceedingly inter- 

 esting to determine precisely what part each of these 

 plays in the aeration of the blood, but so far nothing has 

 been done towards so desirable an end. 



HOMOLOGIES. 



A Polyzooid, reduced to its simplest form, is a closed 

 sac, the walls bent inwards forming an annular fold divid- 

 ing the neural or posterior region from the reproductive 

 or anterior region. The posterior end bears the lopho- 

 phore, a disk bordered by the tentacular organs of respi- 

 ration and prehension, and perforated by a circular eden- 

 tulous mouth, from which hangs the digestive system in 

 the antero-posterior axis of the sac beneath. The alimen- 

 tary canal has a simple, dorsal flexure, the anus opening 

 on the dorsal side near the mouth. 



Animal compound ; nervous system compound, each 

 zooid furnished with two large dorsal ganglia ; * principal 

 muscles distributed in pairs on either side, attached pos- 

 teriorly to the alimentary canal and lophophore, and an- 

 teriorly to the walls of the reproductive region ; circula- 

 tion unconfined by special vessels ; reproduction takes 

 place by buds and by the ova. 



Many malacologists consider the Polyzoa more nearly 

 related to the Ascidia than to the Brachiopoda. The 

 class characters of the Brachiopoda, however, and the 

 special homologies, which may be traced between the 

 organs of a Brachiopod and those of a Polyzooid, show 

 closer affinities than exist between the last and an Ascidi- 

 ian. The higher Brachiopoda, such as Terebratula and 

 Khynconella, have the respiratory tentacles similarly sit- 

 uated around the disk, or lophophore, which is perforated 

 at the centre by the mouth, and from which the alimentary 

 canal hangs in the visceral sac beneath, the mouth and 

 anus approximate, the canal having a dorsal flexure. 



The extension of the lophophore into two or three spi- 

 riform arms, the complex structure of the tentacles and 



* The existence of a complete nervous collar is doubtful. 



