386 ZOOL007. 



Class I. — Tunicata {Ascidians, Sea Squirts). 



General Characters of Tunieates. — These animals were 

 ouce regarded as mollusks, and in former editions of this book 

 they were assigned a position among the worms, between the 

 Brachiopods and the Nemertina. 



Recent advances in our knowledge of Ascidians on the 

 one hand, and of the primitive features of the Vertebrates on 

 the other, show quite conclusively that the Ascidians, par- 

 ticularly the adult form Appendicularia, and the larvse of 

 those Ascidians which undergo a metamorphosis, have the 

 fundamental characters of Amphioxus and the embryos of 

 genuine Vertebrates, such as the lamprey. 



It will be remembered that these fundamental characters 

 are the presence of a notocord, over which lies the central 

 nervous system. No invertebrate is known to possess 

 this dorsal position of the nervous system to the dorsal cord, 

 unless we except Balanoglossus, which, as Mr. Bateson has 

 shown, has a notocord lying under a central nervous cord. 

 If the larva of this form was not like that of the worms and 

 Echinoderms, presenting no vertebrate features, we might 

 adopt Bateson's view that Balanoglossus should be placed 

 at or near the base of the Vertebrate series, in a group Pro- 

 tochordata. 



The result of admitting the Tunieates into the same 

 branch or type as the Vertebrates has led to the proposal of 

 a group Chordata, including the Tunieates and the genuine 

 Vertebrates; but as Amphioxus seems to be a connecting- 

 link between the Tunieates and the genuine Vertebrates, 

 begmning with the hag-fish and the lamprey, we will, for 

 convenience, retain the familiar word Vertebrata for all ani- 

 mals having a notocord situated between a neural and an 

 enteric cavity. 



Fig. 386' will show the close resemblance of the larval as- 

 cidian to the embryo lamprey. 



It will be seen that even the larval Ascidian has an incipi- 

 ent brain, consisting of two ganglia, from whicli arise a 

 spinal nervous cord, with even spinal nerves. The intestine 

 in the lai'val Ascidian is bent and ends in front, but in the 

 adult tadpole-shaped Appendicularia the end of the intes- 

 tine is ventral and opens directly outwards. 



