THE ARTHROPODA OF THE DEEP SEA 147 



The free-swimming Tunicata included in the 

 group Ascidice salpiformes, which contains the genus 

 Pyrosoma, and the order Thaliacea containing the 

 salps, are in all probability mainly confined to the 

 surface waters. A few specimens of Pyrosoma were 

 captured by the ' Challenger ' dredges which came up 

 from very deep water, but it is doubtful at what point 

 in the journey to the surface the specimens entered 

 the net. 



The most remarkable form of free-swim ruing 

 Tunicate that has come to light is Octacnemus bythius, 

 a form that is probably allied to Salpci. It was 

 found twice, once in the dredge that came from a 

 depth of 1,070 fathoms, and once from 2,160 fathoms. 

 The tunic of the animal is gelatinous and hyaline, 

 but the most important feature it possesses is an 

 imperforate membrane separating the branchial sac 

 from the peribranchial cavity. Octacnemus, in other 

 words, possesses no true stigmata, these structures 

 being represented only by little pits in the walls 

 of the branchial sac. This curious and extremely 

 interesting modification of the respiratory organs 

 points very strongly to the conclusion that Octacnemus 

 is truly a deep-sea animal. 



