404 CHORD AT A. 



They chiefly differ from the Cephalochorda in the simple 

 and doubtfully segmented nature of the mesoderm, involving 

 an absence of nephridial excretory organs and of peri- 

 visceral coelom. Their real relationship to the other class 

 is shown most clearly by the structure of the larval form 

 rom the chordula onwards. In some respects the larval 

 ascidian attains a higher level of chordate structure than 

 Amphioxus, 



Like most sedentary forms the Tunicata show a tendency 

 to reproduction by budding, and to its natural corollary, the 

 formation of colonies. These colonial types are called 

 compound ascidians, the individuals being usually embedded 

 in a common test, and sharing a common atrial cavity. 

 Most are sedentary, but some {e.g.^ Pyrosoma) are pelagic. 

 This compound form is a large bell-shaped organism with a 

 huge atrial cavity in its interior. It is strongly phosphores- 

 cent. Amongst other pelagic forms are Appendictilaria, 

 remarkable for retaining its notochord down the centre of a 

 vibratile tail throughout life and possessing a number of 

 other simple features and Salpa, which shows a well-marked 

 metagenesis or alternation of generations. 



Class II. — Cephalochorda. 



This class contains only Amphioxus and a few other 

 genera which do not differ essentially from it. Hence the 

 characters of the class are those of the type. We may 

 specially notice as differences from Vertebrata the produc- 

 tion of the notochord to the extreme anterior end of the 

 body, the absence of paired sense-organs, of a median heart 

 and of jaws, the different method of feeding therein involved, 

 and the whole structure of the pharynx and atrium. 



On the other hand it approaches the Vertebrata nearer 

 than do the Tunicata^ in the structure of the mesoderm, 

 highly developed into segmented myomere muscles, a peri- 

 visceral coelom and numerous nephridial excretory organs, 

 the definite direction of circulation in the blood-vascular 

 organs, andCthe clear indication of a hepatic-portal system. 



In a very general way the method of locomotion is 

 vertebrate and the method of feeding atriozoan. 



