A Guide to the Zoological Collections 



fact that breathing is effected in Tunicates, as in Am- 

 phioxus, by special modifications of the pharynx, has 

 now led to the advancement of the Tunicata to the 

 Vertebrate phylum. 



In Ray Lankester's classification they stand as an 

 independent branch of that phylum, under the name 

 Urochorda, which expresses the fact that the embryonic 

 and larval notochord is confined to the tail. 



The Tunicata, or Ascidians, or Sea-squirts, occur to 

 ordinary observation under two forms : namely, fixed 

 Tunicates that are found encrusting rocks, etc., in shallow 

 water near shore, and free-swimming Tunicates that 

 float at and near the surface of the open sea. Some of 

 the latter are very brightly phosphorescent, e.g., the 

 colony of Pyrosoma in Case 2. 



In both forms the animal is usually encased in a 

 thick gelatinous or leathery tunic (hence the name 

 Tunicata), in which there are two openings : namely, 

 one at the fore-end, leading into the body, and corres- 

 ponding to the mouth of Vertebrate animals ; the other 

 either on the dorsal side, or at or near the after end, and 

 leading out of the body. 



In the fixed Tunicates the test is either semi-trans- 

 parent or opaque, either gelatinous or leathery, and 

 often coloured, and has its two openings rather close 

 together: it often resembles a small leather bag — 

 hence the name ascidian, from do-Kos a leather bag, and 



etSos form. 



In the free-swimming Tunicates the test is trans- 

 parent and gelatinous, and has its two openings either 

 at or almost at the opposite ends of the body. 



There is another form of free-swimming Tunicata 

 that, on account of its small size, needs careful searching 

 for with a drift-net. In this form, enlarged drawings 



