76 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
This group contains a single family, the Appendi- 
culariide, 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 Oikopleura, 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 Fritillarva, on the other hand, the body 1s 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 insufficiently 
known. 
Order Il. THAULIJACHA. 
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 trunk 
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 of 
