8 A Guide to the Zoological Collections 



corresponding, so far as it goes, with that of Amphioxus 

 and Vertebrate embryos, and — dorsal to the notochord — 

 a nerve tube with the same general correspondences. 

 In the head there is a hollow swelling of the nerve- 

 tube having a fundamental identity with the brain of 

 higher Vertebrates. 



Some of the steps by which the free-swimming larval 

 " Ascidian tadpole " changes into the degenerate fixed 

 Ascidian are shown in a series of enlarged drawings 

 after Ray Lankester. 



2. LARVACEA . 

 [Case 2.] 



As has been already mentioned, there are certain 

 very small Tunicates that swim freely through the open 

 seas, and that keep the tail, though not quite in the 

 tadpole form, throughout life. 



Enlarged drawings and models of some of these are 

 shown in Case 3. They have no peribranchial cavity, 

 and the branchial sack therefore opens directly to the 

 exterior, — by two pores or gill-slits. 



3. COMPOUND ASCIDIACEA. 



[Case 2.] 

 Certain Ascidians that resemble in their general 

 anatomy the form shown in our dissections, are able to 

 form colonies by budding. They are represented here 

 chiefly by glass models. Some of these compound 

 Ascidians, as the Pyrosoma in Case 2, are free-swim- 

 ming inhabitants of the open seas. 



4. THALIACEA. 



[Case 2.] 

 Salpa y of which glass models and spirit specimens, 

 and Thalia, of which stained glycerine specimens are 



