CLiSSinCATIUN OF THE TUNICATA. 649 



merely imperforate pits in its walls, and no direct connection 

 with the peribrancliial cavity. 



Dorsal lamina unrepresented. 



Alimentary canal coiled up along with the reproductive organs 

 to form a visceral mass (the " nucleus "), placed at the dorsal edge 

 of the posterior end of the body. 



Gonads hermaphrodite. 



Tliis genus contains the very aberrant abyssal form Octacnemus 

 hythius, Moseley, obtained during the ' Challenger ' Expedition in 

 the South Pacific from depths of 1070 and 2160 fathoms. 



Order III. LARVACEA, Rerdman, 1882. 



The Larvacea, or Copelata(Gegenb.), are free-swimming pelagic 

 forms, provided with a large locomotor appendage (the " tail "), in 

 which there is a skeletal axis (the urochord). 



A relatively large test (the " Haus ") is formed with great 

 rapidity as a secretion from the surface of the ectoderm ; it is, 

 however, merely a temporary structure which may be cast off and 

 afterwards replaced by another. 



The branchial sac is simply an enlarged pharynx with two 

 ventral ciliated openings (stigmata) leading to the exterior. 

 These open independently on the ventral surface. Tht're is no 

 separate peribranchial cavity. 



The nervous system consists of a large anterior :ind dor- 

 sally placed ganglion, and a long nerve-cord with smaller ganglia, 

 stretching backwards fix)m it over the alimentaiy canal to reacli 

 the tail, along which it runs on the left side of the ur'ochord. 



The alimentary canal lies behind the branchial sac, and the 

 anus opens ventrally on the surface of the body in front of the 

 stigmata (or atriopores). 



The gonads are placed at the posterior end of the body. 



Gemmation does not take place, and alternation of generations 

 and metamorphosis do not occur in the life-history. 



This group contains a single family, the AppendiculariiJae, all 

 minute tailed free-swimming forms, which have undergone com- 

 paratively little degeneration, and consequently correspond more 

 nearly to the tailed-larval condition than to the adult forms of 

 the other groups of Tunicata. 



46* 



