' 



640 PEor. w. A. heedman's eevised 



Gonads hermaplirodite, placed posteriorly to the brancliial sac, 

 in diverticula of the peribranchial cavity. 



Gemmation from a ventrally-placed stolon formed at the 

 posterior end of the endostyle. 



The following is a scheme of the knovs^n species of Fyrosoma. 

 P. atlanticum and P. giganteuon are, hovrever, such closely related 

 forms, that a number of characters have to be taken into account 

 in distinguishing between them. I know of no one good 

 character by which these two species can be separated : — 



{Ascidiozooids arranged in regular verticils. 

 P. elegans, Les. (Atl., Med.) 

 Ascidiozooids arranged irregularly 1 



C Surface of tlie colony provided ■with short sharp spines only. 



P. sjnnosu7n, Herdm. (Atl.) 

 Surface of the colony provided with large processes of the test 2 



{Colony conical, processes of test subulate or conical. 

 P. atlanticum, Peron. (Atl.) 

 Colony cylindrical, processes of test flattened and lanceolate at their 

 free ends P. giganteum*,'Le~. (Atl., Med., 



[A. Arct., Mai.) 



Order II. THALIACEA {Sav.), v. d. Hoeven. 



The Thaliacea are free-swimming pelagic forms, which 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 com- 

 municates with the exterior by the atrial aperture. 



The anus opens into the peribranchial cavity. 



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

 be complicated by polymorphism. 



This order includes all the pelagic Tunicata with the exception 

 of Fyrosoma and the Appendiculariidse. It seems a natural 



* It must be remembered that P. giganteum is not the largest species. 

 P. simiosum, discovered during the ' Challenger ' Expedition, is many times 

 larger than Lesueur's species. 



