CHAPTER II 



CLASSIFICATION AND DEVELOPMENT 



Classification of Tunicates. Order I. Appendicularians. Order II. As- 

 cidians. Order III. Salpians. Numbers, relationships. Life-history of a 

 typical Ascidian. 



T 



HE Class Tunicata is divided into three orders: the Ap- 

 pendicularians, the Ascidians, and the Salpians. 



Order I. Larvacea or Appendicularians 



These are, with one or two exceptions, pelagic forms, usually 

 of small size (about 5 mm. in length), swimming actively by 

 means of a relatively large tail which is supported by the noto- 

 chord. (Plate XXXV., A.) They retain throughout life what 

 the Ascidians lose at metamorphosis — a long dorsal nerve-cord, 

 a notochord, and primary gill-slits (one pair). The food-canal 

 ends ventrally ; there is no peribranchial cavity. They are 

 able to form very rapidly a cuticular test or " house " many 

 times larger than the body, which is from time to time thrown 

 off and replaced. 



Like many pelagic animals, they are usually transparent but 

 some have brightly coloured spots, such as the reproductive 

 organs ; they have a very wide representation — in all seas, and 

 some particular forms have a great range of distribution ; and 

 they often occur in great swarms, forming especially in summer 

 an important constituent in the surface " plankton," or float- 

 ing life. 



About forty species of Larvacea are known, distributed in 

 about ten genera, e.g., Appendicularia, Oikopleura (British), 

 Fritillaria (British), Kowalevskia, and Megalocercus, The 

 genus Kowalevskia is somewhat apart from the others, e.g., in 

 the absence of heart and endostyle. The largest known Ap- 

 pendicularian is Megalocercus abyssorum from deep water in 



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