26 



THE VOYAGE OF H.M.S. CHALLENGER. 



The closed end of the colony is formed by one central Ascidiozooid witli six others 

 arranged around it. After that the Ascidiozooids are arranged quite irregularly, but 

 are all placed with their ventral surfaces towards the closed end of the colony, so that 

 the common cloacal aperture is situated dorsally to the Ascidiozooids. 



A small colony obtained off the coast of Africa on August IG, 1873, is probably 

 also referable to this species ; but it is young, and the characters are not well marked 

 (see p. 33). 



Pyrosoma giganteum, Lcsueur (PI. I. figs. 4-21). 



Pyrosoma giganteum, Lesueur, Journ. de Physi(]ue, 1815, p. 413. 



This is the species of Pyrosoma which has been the most thoroughly investigated. 

 It was described and well figured by Lesueur in 1815. It was independently investi- 

 gated at the same time by Savigny, who gave an account of its anatomy in 1816. It 

 underwent further investigation at the hands of Keferstein and Ehlers in 1861 ; and, 

 finally, it was the subject of Huxley's careful and detailed researches made in the 

 following j'ear. Each of these distinguished naturalists added somewhat to the know- 

 ledge gained by his predecessors. 



One specimen of this species was obtained l)y the Challenger Expedition in the 

 surface-net off" Cape Verde in August 1873. It is of cylindrical form, and has the 

 terminal opening very small. 



The measurements are as follows : — 



Total length, 



Breadth near open end, . 



„ at thickest part (near middle), 



,, at closed end. 



10-5 cm. 

 1*5 cm. 

 2 '5 cm. 

 1'5 cm. 



Diameter of cloacal cavity, close to opening, 1 -4 cm. 



,, ,, ,, near middle, . 1-0 cm. 



,, of common cloacal aperture, . 04 cm. 

 Thickness of colony, . . about 0'3 to 0"4 cm. 



The Ascidiozooids are placed just as in the case of Pyrosoma atlanticum, with their 

 ventral surfaces towards the closed end of the colony, and the long processes of the test 

 on the outer surface are placed on the dorsal edges of the branchial apertures, and are 

 therefore turned towards the open end of the colony (Fig. 5). 



cl .c 



Fig. 5. -Diagram showing the relation of the Ascidiozooiil, and of the process of the test to the closed 



end of the colony in Pitrosoma fjigantcum, 



at., atrial aperture ; Ir. branchial aperture ; c.c. common cloaca of the colony ; ci.e. closed end of the colony ; end. endostyle 



indicating the ventral edge of the Ascidiozooid ; t.p. process of the test ; w. thickness of the colony. 



