OCTACNEMUS 



109 



genus Octacnemus, now known in a solitary and an aggregated 

 form. It was found during the " Challenger " expedition, and was 

 first described by Moseley. It is apparently a deep-sea repre- 

 sentative of the pelagic Salpidae, and may possibly be fixed at 

 the bottom. The body in the solitary form is somewhat discoid, 

 with its margin prolonged to form eight tapering processes, on to 

 which the muscle-bands of the mantle are continued. The 

 alimentary canal forms a compact nucleus, which is attached to 

 an apparently imperforate membrane which stretches across the 

 body, separating the branchial from the atrial cavities. The 



end,, br 



FlO. 68. — A, solitary form of Octacnemus bythius (after Moseley) ; E, diagram of struc- 

 ture of Octacnemus (after Herdmaii) ; C, aggregated form of 0. patagoniensis (after 

 Metcalf). 1, from outside ; 2, with test removed ; and 3, with mantle removed. 

 a. Anus ; adh, area of attachment ; at, atrial, and br, branchial aperture ; hr.s, 

 branchial sac; eM(i, endostyle ; g.s, gill - slits ; i, intestine; n.y, nerve-ganglion; 

 oe, oesophagus ; ov, ovary ; p.br, peribranchial cavity ; st, stomach ; stol, stolon. 



endostyle is very short, and the dorsal lamina is also much 

 reduced. The reproduction and life-history are entirely unknown. 

 The aggregated form consists of a small number of individuals 

 united by a slender cord composed of test, body -wall, and endo- 

 dermal tissue. Octacnemus has been found ^ in the South Pacific 

 from depths of 107 and 2160 fathoms, and off the Patagonian 

 coast from 1050 fathoms. Two species have been described: 0. 

 hytUus, Moseley, and 0. patagoniensis, Metcalf. Metcalf, who 

 has recently investigated the aggregated form (0. patagoniensis), 

 considers that the genus is more nearly related to the Clavelinidae 

 than to the Salpidae. Possibly its position might be best 



1 See Herdman, Challenger Report on Tunicata, part iii. 1888, p. 88 ; and Met- 

 calf, Johns SopTcins Univ. Circ. No. 106, 1893, and Zool. Jahrb. AUh. Anat. 

 xiii. 1900, p. 572. 



