APPENDIX. 



It may be useful to add here a brief statement of the 

 classification and characters of the Tunicata, in order to 

 indicate the position of AscicUa as a type of the group, 

 and its relations to the other British Ascidians.* 



TUNICATA. 



The Tunicata (or Urochorda) are hermaphrodite marine 

 chordate animals, which show in their development the 

 essential vertebrate characters, but in which the notochord 

 is restricted to the posterior part of the body, and is in 

 most cases present only during the free-swimming larval 

 stage. The adult animals are usually sessile and degener- 

 ate, and may be either solitary or colonies, fixed or free. 

 The nervous system is in the larva of the elongated, 

 tubular, dorsal vertebrate type, but in most cases degener- 

 ates in the adult to form a small ganglion placed above 

 the pharynx. The body is completely covered with a thick 

 cuticular test ("tunic "), which contains a substance similar 

 to cellulose. The alimentary canal has a greatly enlarged 

 respiratory pharynx (the branchial sac), which is perforated 

 by two, or many, more or less modified gill slits, opening 

 into a peribranchial or atrial cavity, which communicates 

 with the exterior by a single dorsal exhalent aperture, 

 rarely two ventral apertures. The ventral heart is simple 

 and tubular, and periodically reverses the direction of the 

 blood current. 



* For a more detailed classification, with definitions of all the groups and 

 analytical keys to the species, see Herdnian's Revised Classification of the 

 Tunicata. .lourn. Linn. Soc. -Zool., vol. XXIII , p. 558, 1S91. 



