ASCIDIA. 77 
generations occurs in the life-history, and may be compli- 
cated by polymorphism. The order Thaliacea comprises 
two groups, CYCLOMYARIA and HEMIMYARIA. 
Sub-order I. CYCLOMYARIA. 
Free-swimming pelagic forms, which exhibit alternation 
of generations in their life-history, but never form perman- 
ent colonies. The body is cask-shaped, with the branchial 
and atrial apertures at the opposite ends. The test is 
moderately well developed. The musculature is mostly in 
the form of complete circular bands surrounding the body. 
The branchial sac is fairly large, occupying the anterior 
half or more of the body. Stigmata are usually present 
in its posterior part only. The peribranchial cavity is 
mainly posterior to the branchial sac. The alimentary 
canal is placed ventrally close to the posterior end of the 
branchial sac. Hermaphrodite reproductive organs are 
placed ventrally near the intestine. 
This group contains only one British genus, Doliolum, 
and even that is probably only an occasional visitant. It 
has a cask-shaped body, from 1 to 2 cm. in length, with 
lobed terminal branchial and atrial apertures. The body- 
wall contains eight or nine circular muscle bands, by the 
contraction of which the animal swims. 
The best-known British form, Doliolwm tritonis, has 
been captured on occasions in thousands off our N.W. 
coast, between the Hebrides and the Faroes. This species, 
and the closely allied D. nationalis, have also been found 
in the English Channel and off the S.W. coast of Ireland. 
Sub-order I]. HEMIMYARIA. 
Free-swimming pelagic forms, which exhibit alternation 
of generations in their life-history, and in the sexual con- 
- dition form colonies, The body is more or less fusiform, 
