GENERAL INTRODUCTION 23 



structure of the animal is totally unlike that of the bivalve 

 molluscs. The Tunicata are exclusively marine and com- 

 prise many animals of very different appearance. There 

 are the common sea squirts of our shores and shallow seas, 

 either solitary creatures like little gelatinous or leathery 

 bags fastened on to rocks, or else great numbers of smaller 

 individuals embedded in a common gelatinous mass, solitary 

 and colonial Tunicates respectively. Each individual has 

 two openings which, when the animals are squeezed, squirt 

 out a stream of water, hence their common name. Other 

 varieties of Tunicates, called Salps and Appendicularians, 

 form an important part of the drifting life of seas somewhat 

 warmer than our own, such as the Mediterranean. They 

 are transparent animals, which may be solitary but are 

 often fastened together in long chains. 



In their early stages the Tunicates resemble little tadpoles 

 with a backbone which later disappears but the possession 

 of which may mean that they are really very degenerate 

 relations of the Vertebrates — distinguished from the 

 Invertebrates, which we have hitherto been considering, 

 by the possession of a backbone. The simplest animals 

 to show certain vertebrate characteristics are the Lancelets, 

 like little white eels which burrow in the sand, while, rather 

 simpler than the true fish, are the Cyclostomata, of which 

 the lamprey is our chief example. They have a larger 

 number of gill openings than the true fish, have round 

 mouths with no true teeth and have no paired fins on the 

 sides of the body. 



The fish or Pisces are too well-known to need description. 

 They are divided into two principal groups, one, called the 

 Elasmobranchs, having a relatively soft, cartilaginous 

 skeleton and with the gill openings separate — to mention 

 two of their chief characteristics — and including the dog-fish, 

 sharks and skates, and the other, known as the Teleosts 



