41 



generation!? occurs in the life-history, and may be compli- 

 cated by polymorphism. The order Thaliacea comprises 

 two groups, Cyclomyaria and Hbmimyaria. 



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 ventral ly close to the posterior end of the 

 branchial sac. Hermaphrodite reproductive organs are 

 placed ventral ly near the intestine. 



This group contains only one British genus, Duliolum, 

 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 tlie animal swims. 



The best-known British form, Doliolum 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 II. 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. 



