PYROSOMA. 



39 



Eur. and Med. Rhopalaea Philippi, zooids few, not united by tests, stolons 

 as foliaceous expansions ; 5-12 cm. ; Med. Rhopalopsis Herd. 



Tribe 3. ASCIDIAE SALPAEFORMES (ASCIDIAE LUCIAE). 

 Free-swimming pelagic colonial Ascidians. 



The colonies have the form of hollow cylinders, closed at one 

 end, open at the other, and slightly 

 tapering towards the closed end. The 

 closed end is rounded ; the open end 

 is flat and its edges project inwards 

 to form a diaphragm (Fig. 32). The 

 cavity of the cylinder is the common 

 cloaca of the colony, and its opening, 

 which can vary in size, is the common 

 cloacal opening. The test is trans- 

 parent and gelatinous ; it bears on its 

 outer surface a number of projections 

 (Fig. 31 bis], while the inner surface is 

 perfectly smooth. The zooids are elon- 

 gated antero-posteriorly and placed in 

 the thickness of the wall of the cylinder 

 in a single layer at right angles to the 

 surface. Their mouths open on the 

 outer surface, each at the base of one of 

 the test processes ; and their atrial 

 apertures are at the opposite end and 

 open into the common cloaca. The 

 colonies vary in length from a few 

 inches to four feet (Pyrosoma spinosum). 

 They float horizontally in the sea, and 

 have a slight power of movement with 

 the closed end forward. The movement 

 appears to be caused by feeble longi- 

 tudinal contractions of the wall of the 

 hind end of the cylinder. They are 

 phosphorescent. When they are at rest 

 the light is very feeble, but on stimulation by contact or other 

 means, the light, passing through a red and green stage, be- 

 comes white. Moseley * states that he traced his name on the 



* Notes by a Naturalist on the " Challenger" London, 1879, p. 574. 



FIG. 31 Ins. Pyrosoma elegant, 

 x i (from Perrier). a cloacal 

 opening ; d buccal appendages 

 of the zooids. 



