468 



The Tunicates, or Ascidians 



The following account of the structure and development 

 of the Tunicate is taken, with considerable modiflcafon and 

 condensation, from Professor Kingsley's chapter on the group 

 in the Riverside Natural History. For the changes suggested 

 I am indebted to the kindness of Professor William Emerson 



Ritter : 



The Tunicates derive their name from the fact that the 

 whole body is invested with a tough envelope or " tunic." This 

 tunic or test may be either gelatinous, cartilaginous, or leathery. 

 In some forms it is perfectly transparent, in others it is trans- 

 lucent, allowing enough Hght to pass to show the colors of the 

 viscera, while in still others it is opaque and variously colored. 

 The tunic is everywhere only loosely attached to the body 

 proper, except in the region of the two openings now to be 

 mentioned. One of these openings occupies a more or less 

 central position, while the other is usuahy at one side, or it may 

 even be placed at the opposite end of the body. On placing 

 one of the Ascidians in a glass dish and sprinkling a little car- 

 mine or indigo in the water, we can study some of the func- 

 tions of the animal. As soon as the disturbance is over, the 

 animals will open the two apertures referred to, when it will 

 be seen that each is surrounded with blunt lobes, the number 

 of which varies with the species. As soon as they are opened a 

 stream of water will be seen to rush into the central opening, 

 carrying with it the carmine, and a moment later a reddish 

 cloud will be ejected from the other aperture. From this we 

 learn that the water passes through the body. Why it does so 

 is to be our next inquiry. On cutting the animal open we find 

 that the water, after passing through the first-mentioned open- 

 ing (which may be called the mouth) enters a spacious cham- 

 ber, the walls of which are made up of fine meshes, the whole 

 appearing like lattice-work. Taking out a bit of this network 

 and examining it under the microscope, we find that the edges 

 of the meshes are armed with strong cilia, which are in constant 

 motion, forcing the water through he holes. Of course, the 

 supply has to be made good, and hence more water flows in 

 through the mouth. This large cavity is known as the branchial 

 or pharyngeal chamber. It is, according to Professor Ritter, 

 "as we know from the embryology of the animal, the greatly 



