REPORT ON THE TUNICATA. 



25 



Diplosomida) amongst the Compound Ascidians, and may possibly occur also in otliers. 

 Since, then, the two remarkable processes in the life-history of Pyrosoma, the asexual 

 production of Ascidiozooids from a ventral stolon, and the embryonic blastogenesis in 

 the sexual reproduction, are both found amongst normal Compound Ascidians ; and 

 since the remarkable shape of the colony, and of the adult Ascidiozooids, is seen in the 

 almormal Compound Ascidian Ccelocormus huxleyi, there can, I think, Ije little doubt 

 that Pyrosoma is related to the Ascidiaj Composita;, and that Ccelocormus occupies a 

 position between the two grouj)s. 



The following is a scheme of the species in this genus. Pyrosoma atlanticum and 

 Pyrosoma giganteum are, however, 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 species can be separated : — 



Pyrosoma. 



Ascidiozooids arranged 

 in regular verticils. 



I 

 Pyrosoma dedans. 



Ascidiozooids arranged 

 irregularly. 



Surface of the colony 

 provided with large 

 processes of the test. 



Surface of the colony 



jjrovided with short 



sharp spines only. 



Pyrosoma sphwsum. 



Colony conical, 



processes of test 



subulate or conical. 



I 



Pyrosoma atlantkum. 



Colony cylindrical, 



processes of test 



flattened and lanceolate 



at their free ends. 



Pyrosoma giganteum. 



Pyrosoma atlanticum, Peron (PL I. figs. 1-3). 



This is the original species described first by Peron in 1804. It is also probabl}' the 

 species investigated by Huxley in 1851. For the characters by which it may be distin- 

 o-uishecl from the other two older species, we are indebted to Lesueur and to Savigny. 



One specimen of Pyrosoma atlanticum was obtained by the Challenger Expedition 

 in the surface-net off Cape Yerde in August 1873. It is of regular conical form, tapering 

 towards the closed end of the colony. It is of a transparent sliglitly bluish grey colour. 



The measurements are as follows : — 



Total length, .... 



Breadth close to open end, . 



Do. at closed end. 

 Diameter of common cloacal aperture. 

 Thickness of colony from outer to inner surface, 



(ZOOL. CHALL. EXP. — PART LXXVI. — 1888.) 



9-5 cm. 

 3-0 cm. 

 0-8 cm. 

 1-3 cm. 

 0-3 cm. 



GacTft . 



