REPORT ON THE TUNICATA. 277 



from the centre of the Pacific Ocean, on the equator. It is a curious fact that not more 

 than one of the species was obtained at any single locality. The only two from the same 

 neighbourhood are Culeolus recumbens and Culeolus perlucidus, which were obtained at 

 consecutive Stations (Nos. 146 and 147) in the Southern Ocean. 1 The first of these 

 stations is the most southern locality for the genus, while Station 44, the locality for 

 Culeolus perlatus, is the most northern extension. 



Culeolus wyville-thomsoni, which is by far the least deep form of the genus, has a 

 thicker and more ordinary looking test than any of the other species. Otherwise the 

 depth seems to have no effect, the two deepest forms, Culeolus murnvji and Culeolus 

 moseleyi, having thick, opaque tests, while much the most fragde and transparent form 

 is Culeolus perlucidus, from the intermediate depth of 1600 fathoms. 



It seems impossible to establish any relation between the nature of the bottom and 

 the occurrence of this genus. Three of the localities are marked globigerin a ooze. Two 

 of these, Stations 146 and 147, are very pure and typical examples of this deposit, while 

 the third, Station 271, has a considerable admixture of Radiolarians and Diatoms. Of the 

 three remaining localities, one, Station 241, is a red clay; another, Station 170, is a 

 volcanic deposit, composed of fragments of rock and pumice, with a little mud ; while the 

 last, Station 44, is a blue mud, formed of continental debris. 



The most important morphological peculiarity is undoubtedly the very remarkable 

 condition of the branchial sac, which is found in all the species of Culeolus {e.g., PL 

 VIII. fig. 3), in Fungulus ctnereus (PL XIII. fig. 9), and in the curious little species 

 Bathyoncus mirabilis (PL XXIV. fig. 9.) one of the Styelinas, but also a deep-water 

 form. It is quite distinct from the branchial sac in any other known Simple Ascidians, 

 and it is interesting to find it present in a member of a different sub-family. This 

 peculiar and simple structure, in which stigmata are apparently not formed, in consequence 

 of the suppression of the fine interstigmatic vessels, at first naturally suggests the 

 simple mesh-work found in Pyrosoma; but I am inclined to believe that the true structure 

 of the sac in that genus is a double row of laterally placed stigmata, running transversely 

 in place of longitudinally, and crossed at right angles by the internal longitudinal Inns. 

 In this case the branchial sac of Pyrosoma shows a simplified state of the condition found 

 in Boltenia elegans, 2 where the stigmata are transverse (PL VII. fig. 2.), and is entirely 

 different from the branchial sac of Culeolus. Consequently, lam not of opinion that the 

 simple form of sac seen in Culeolus, Fungulus and Bathyoncus is a primitive form which 

 has survived, but think, on the contrary, that it is an after modification of a more compli- 

 cated type, which has probably taken place independently in the Bolteninaa and Styelinae, 

 and after the separation of these two groups by the development of compound tentacles 



i And Culeolus murrayi, and the new species from Station 241. 



2 I very much doubt even this being the survival of a primitive character, as Boltenia is certainly a highl) modified 

 form. 



