PRIMITIVE VERTEBRATES 293 



however, as Lung- Fishes, Chimaera, Lamprey, and Hag, it per- 

 sists throughout life, though invested in a firm sheath and supple- 

 mented by the development of cartilage. Ordinary bony fishes 

 are a good instance of partial persistence of the notochord. 

 Almost everyone must have noticed, in the pursuit of breakfast- 

 table anatomy, that a gelatinous substance occupies the spaces 

 between the doubly-cupped ends of the vertebrae in such fish 

 as salmon and cod. This substance represents the part of the 

 notochord which has not been nipped out of existence by the 

 ingrowth of hard material to make up the joints of the backbone. 

 In a Protochordate there never is a vertebral column or a back- 

 bone in the proper sense of the word, but only a more or less 

 developed notochord. 



2. It is scarcely less distinctive of Chordate animals that in 

 them the pharynx should be perforated by visceral clefts in the 

 embryo if not in the adult. 



3. It is typical for a Chordate to possess a central nervous 

 system (brain and spinal cord) situated dorsally above the noto- 

 chord, and having the nature of a thick- walled tube. 



We will apply these tests to the three recognized orders 

 of Protochordates, i.e. : i. Lancelets (Cephalochorda) ; 2. Sea- 

 Squirts (Urochorda); and 3. certain still simpler forms grouped 

 provisionally as Hemichorda. Very great theoretical interest 

 attaches to the study of these forms, and much attention has 

 been bestowed upon them, largely with a view of finding out 

 how far they throw light upon the obscure problem having for 

 its goal a determination of the characters of the simplest and 

 earliest Vertebrates which appeared upon the globe. 



SUB-CLASS i. LANCELETS (CEPHALOCHORDA) 



Lancelets are small fish -like creatures widely distributed 

 round the coasts of the globe where the two conditions of sand 

 and shallow water are combined. There are about 8 species, 

 popularly referred to one genus Amphioxus (Gk. amphi, both; 

 oxus, sharp), which, like the ordinary name, has reference to the 

 fact that the flattened body is pointed at both ends. The common 

 European species (Amphioxus lanceolatus) (fig. 17) ' 1S found 

 on the Mediterranean and both Atlantic coasts, including our 

 own islands. It is particularly abundant at Naples, where the 



