164 



THE VOYAGE OF H.M.S. CHALLENGER. 



some of the species. In Culeolus murrayi the terminal twigs of the vessels open 

 into large vesicles placed just below the surface of the test, and only separated from the 

 external medium by a very delicate membrane. Iu several of the species there are 



Fig. 66. — The Branchial Sac of Culeolus wyville-thomsoni, Herdman, from the inside; magnified about 50 diameters. 

 tr., large transverse vessel ; tr 1 ., smallest size of transverse vessel ; i.l., internal longitudinal bar ; mh., mesh ; sj>., spicules. 



thin-walled hollow papillae or projections from the surface of the test, and these are in 

 free communication w r ith either the large vesiscle or the ends of the vessels. This is 

 obviously an accessory respiratory apparatus, permitting the blood circulating in the 



test (which when the heart contracts dorso-ventrally is 

 impure) to be brought into such close relation with the 

 external water as to ensure a certain amount of oxi- 

 dation. 1 



" A large number of other new species of Cynthiidas 



were obtained, but the only other one which cannot be 



referred to a known genus is Bathyoncus mirabilis, a 



form which agrees with the typical Styelinae in having 



simple tentacles, but differs from them in having a branchial 



sac of the skeleton type found in Culeolus and Fungulus. 



The large and well-marked genus Styela is remarkable 



on account of its very extended bathymetrical range. 



Most of the species are found in shallow water, some few 



between tide marks; while six species in the collection are from between 100 and GOO 



fathoms, and two, Styela bythia and Styela squamosa (see fig. 67), both fairly typical 



members of the genus, were obtained at a depth of 2600 fathoms. 



1 For further details the reader is referred to the Report on the Tunicata, Zool. Chall. Exp., part xvii. pp. 93, 276. 



Flo. 67.— A, Styela squamosa, Herdman, and 

 B, Styela bythia, Herdman (natural size), 

 attached to a manganese nodule, from 

 2600 fathoms. 



