170 



MOLLrSCA. 



Class VII. TUNICATA. 



SACCOPHORA. ASCOZOA. PUOTOVERTEBRATA. UROCIIORDA. 



Marine, simple or compound, unsymmetrical animals, pro- 

 tected by a coriaceous sac, or, in the compound, jelly-like skin, 

 with two apertures [oral and a trial]. No foot. Heart a simple 

 tube. Mostly hermaphrodite. Young tadpole-like, free-swim- 

 ming. 



The nervous system is confined to a single ganglion with its 

 branches. Tbere are no eyes, but one or more pigment-spots 

 have been detected. There are also a liver, stomach, and convo- 

 luted intestine, the former sometimes rudimentary, but usually 

 very large ; in the latter the flexure is hsemal, according to Huxley, 

 not neural as in Polyzoa. 



The Tunicata follow the Mollusca, and are a " typus " or sub- 

 kingdom for Claus. They form Schmarda's twenty-third class, 

 which he places between Polyzoa and Mollusca. Hackel classes 

 them with Vermes, but having a " true blood-relationship" with 

 Vertebrata. Huxley considers them "in many respects" an 

 isolated group. 



There are two orders ; but their limits are not very definite, 

 Pyrosoma and Doliolum being in some respects intermediate. 



A single ribbon-shaped branchia EIPHORA. 



A pharynx acting as branchiae ASCIDIOIDA. 



