THE AFFINITIES OF THE PELAGIC TUNICATES. 



(Presented to the National Academy, November, l'.tt>4.) 



No. i. ON A NEW PYROSOMA. 

 ( Dipleurosoma elliplica. ) 



By William Keith Bhuoks, LL. D., 

 Henry Walters Professor of Zoology in the Johns Hopkins University. 



Part I.— INTRODUCTORY. 



I am indebted to Dr. Caswell Crave for the opportunity to study the Pyrosoma that is here 

 described. The specimens were collected in the Gulf Stream oti' Beaufort, North Carolina, by 

 the United States Commission of Fish and Fisheries, and were brought to the marine laboratory 

 of the Commission at Beaufort. They were intrusted to me for study by Doctor Grave, the 

 director of the laboratory. The illustrations that accompany the memoir were drawn by Mr. 

 Carl Kellner. 



While all the species of Pyrosoma that have been described arc circular in cross section, the 

 cross section of the one that is to be described is a flattened ellipse, so that the colony has two 

 broad sides (tig. 4) and two narrow edges (tig. .">). 



Except for this flattening, it does not differ in any essential way from other Pyrosomus. 

 which are tubular colonial ascidians that float or swim in the water of the ocean, usually at 

 considerable depth below the surface, but often at or very near it. As their name expresses, 

 thev are among the most brilliantly luminous of marine animals, glowing witli an intense white 

 light that is notable even under the noonday sun of the tropical ocean. The light, which is 

 under the control of the organism, is emitted by a pair of luminous organs (tig. 2 and tig. 8, /'). 

 on each side of the pharynx, near the mouth, and in the coelomic cavity. 



The basis or foundation of the colony, that binds the ascidians together into an organized 

 whole, is a hollow tube of cellulose (ties. 2 and 4) (dosed at one end. A. and opened at the other, 

 B. The open end carries a muscular diaphragm, by which the aperture may be reduced or 

 enlarged. 



The ascidian units, or ascidiozooids, many hundreds or thousands in number, are so placed 

 that their mouths </, are on the outer service of the tube, while their cloacal apertures, c, open 

 into the cavity of the tube, or common cloaca (tig. 3, CC), which again opens to the external 

 water through the terminal opening which may be reduced in size. or. perhaps, completely 

 closed by the muscular diaphragm. 



The members of the community breathe and obtain their food, like other ascidians, by draw- 

 ing water through the mouth (tig. 8, d) into the pharynx or gill-chamber (fig. S, <•) bythe vibration 

 of the cilia around the gill slits. The waste water is discharged from the body through the cloaca] 



151 



