40 



This group contains a single family, the Appendi- 

 culariidae, all minute (about 5 mm. long), tailed, free- 

 swimming forms which have undergone comparatively 

 little degeneration, and, consequently, correspond more 

 nearly to the tailed larval condition than to the adult 

 forms of the other groups of Tunicata. There are nearly 

 a dozen genera known, of which at least two, probably 

 more, inhabit British seas. In the genus Oihopleura, to 

 which our commonest Appendicularians belong, the body 

 is short and ovoid, and no anterior fold or "hood" is 

 present. The tail is three or four times the length of the 

 body, and four to six times as long as it is broad. 

 In Fritlllaria, on the other hand, the body is elongated, 

 and somewhat constricted in the middle where the tail is 

 attached. A fold of integument on the front of the body 

 forms a "hood." The tail is short and wide, not twice 

 as long as the body. 



The British species of Larvacea are still insufiticiently 

 known. 



Order II. THALIACEA. 



Free-swimming pelagic forms of moderate size, which 

 may be either Simple or Compound, and in which the 

 adult is never provided with a tail or notochord. Conse- 

 quently the whole body here corresponds to the tnmlc 

 only of the Appendicularian, without the tail. The test 

 is permanent, and may be either well-developed or very 

 slight. In all cases it is clear and transparent. The 

 musculature of the body-wall is in the form of more or 

 less complete circular bands, by the contraction of which 

 water is ejected from the body, and so locomotion is 

 effected. The branchial sac has either two large or many 

 small apertures (stigmata), leading to a single peribranchial 

 cavity, into which the anus also opens. Alternation pf 



