332 C. ROUSSELET ON PEROPHORA LISTERI. 



place, although, of course, its structure is perfectly well known, 

 and I have nothing to add that is new. 



Mr. Michael some years ago read before you a paper " On the 

 Larva of an Ascidian found at the Land's End,"* illustrating it 

 with some slides he had mounted of the embryo in various 

 stages. He then gave you an interesting account of the tad- 

 pole-like, free-swimming stage through which most of the 

 young Ascidians pass, and of their supposed affinity to the 

 vertebrated animals, on account of the formation of a primitive 

 neural canal, and of a rod-like body in the tail, which is held 

 by eminent zoologists to correspond to the notochord of the 

 vertebrata, and, if true, would raise the tunicates from the 

 mollusca, to which they have previously been classed, to the 

 lowest representative of the vertebrata. 



I will not, however, discuss this subject, but only draw your 

 attention to the interest that is attached to the Ascidian as a 

 class, and will confine my remarks to a short description of the 

 sj^ecimen before you. 



Perophora, first discovered by Mr. Lister, is one of the 

 " Social " Tunicates as distinguished from those that are 

 " simple ' on the one hand, and those that live aggregated 

 together under the same tunic, and are called " compound." 



It is found growing on sea-weeds, not unlike some of the 

 hydroid Zoophytes. It can easily be seen as a cluster of 

 minute globules of jelly, each separate on a peduncle, but all 

 organically connected together by a creeping stalk or stolon. 

 In size the individuals average ~ inch, and when clean their 



O l b ' 



test is so transparent that all the organs can be seen, and the 

 structure studied with the greatest ease under the microscope. 



The test or tunic is nearly square in shape, with two large 

 apertures situated close to one another at the free extremity. 

 One of these is the oral aperture, or mouth, leading into the 

 branchial sac and alimentary canal, whilst the other is the anal 

 funnel. The test is leathery in consistency, and remarkable as 

 being one of the few animal structures containing cellulose. 



Below the test there is another membrane, the second tunic, 

 or mantle, only loosely connected with the first, which is mainly 

 composed of longitudinal and transverse muscular fibres, by 

 means of which the animal can contract with considerable 



* " Journ. Q.M.C.," Ser. II, Vol. 2, p. 111. 



