404. CHORDATA. 
They chiefly differ from the Cepha/ochorda in the simple 
and doubtfully segmented nature of the mesoderm, involving 
an absence of nephridial excretory organs and of peri- 
visceral ceelom. ‘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 Zumicata 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 Appendicularia, 
remarkable for retaining its notochord down the centre of a 
vibratile tail throughout life and possessing a number of 
other simple features and Sa/éa, which shows a well-marked 
metagenesis or alternation of generations. 
Crass 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 Zunicata, 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, and the clear indication of a hepatic-portal system. 
In a very general way the method of locomotion is 
vertebrate and the method of feeding atriozoan., 
