36 THE VOYAGE OF H.KS. CHALLENGER. 



Order II. THALIACEA. 



The Thaliacea are free-swimmiDg pelagic forms, winch may be either simple or 

 compound, and, in the adult, are never provided with a tail or a notochord. 



The test is permanent, and may be either well developed or very slight. 



The musculature of the mantle is in the form of more or less complete circular 

 bands, by the contraction of which locomotion is effected. 



The branchial sac has either two large or many small apertures (stigmata), leading 

 to a single peribranchial cavity which communicates with the exterior liy the atrial 

 aperture. 



The anus opens into the peribranchial cavity. 



Alternation of generations occurs in the life-history, and may be complicated by 

 j)olymorphism. 



This order includes all the pelagic Tunicata, with the exception of Pyrosoma and 

 the Appendiculariidse. It seems a natural, well-defined group, characterised amongst 

 the free-swimming forms by the absence of a tail and a notochord in the adult, and Ijy 

 the occurrence of alternation of generations in the life-history. Temporary colonies 

 may be formed at one stage in the life-history, but they never increase in size by 

 gemmation from the Ascidiozooids, which eventually separate from one another. This, 

 along with the alternation of generations, distinguishes the Thaliacea from the Ascidia3 

 Salpiformes, and the absence of a tail and notochord separates them from the Larvacea. 



The Thaliacea may be divided into two groups, the Cyclomyaria and the 

 Hemimyaeia. 



Suborder I. CYCLOMYAEIA. 



Free-swimming pelagic forms which exhibit alternation of generations in their life- 

 history, but never form permanent colonies. 



The body is cask-shaped, with the branchial and atrial apertures at the opposite ends. 



The test is more or less well developed. 



The mantle has its musculature in the form of 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 jjlaced close to the posterior end of the branchial sac. 



Hermaphrodite reproductive organs are placed ventrally near the intestine. 



This suborder Avas formed by Uljanin^ in 1884 for the reception of the single family 

 ' Fauna und Flora des Golfes von Neapel, Monogr. x., Doliolum. Leipzig, 1884. 



