2 GENERAL SURVEY OF THE ANIMAL KINGDOM. 



But all these hard and fast distinctions are apt to be mis- 

 leading. No zoologist can draw the boundary line between 

 Invertebrates and Vertebrates with a steady hand. We 

 hardly know where the backboned series begins, — whether 

 with the worm-like Balanoglossus and Cephalodiscus, or with 

 the degenerate Tunicata, or with the remarkable lancelet 

 {Amphioxus). Moreover, not a few Invertebrate animals 

 (Nemertean "worms," Chsetopod " worms," and even Crus- 

 taceans) approach Vertebrates in some of their features. 



Before we begin with the Invertebrate animals, we must 

 answer the question, " How do you know wh^e to start ?" 

 In other words, we have to explain the " basis of classifi- 

 cation." We begin with the animals of simpler structure, 

 as we would begin architectural studies with the simplest 

 buildings. We pass thence to more complex forms, and, of 

 course, we together rank those which show a similar style of 

 architecture. To some extent we are helped in our order 

 of procedure by the study of extinct animals and their 

 gradual appearance upon the earth (Palseontology). Sut the 

 oldest rocks with certain and intelligible fossils, contain 

 many types of different degrees of complexity, and we can- 

 not tell from this study alone in what precise order the 

 simpler animals were evolved. We get more help from the 

 study of individual development (Embryology). Thus we 

 learn that most young embryos have the form of a two- 

 layered sac of cells, a gastrula. This gastrula occurs with 

 great constancy in the development of animals, and the 

 conclusion suggested is that the earliest and simplest 

 Metazoa were like gastrulse. 



Classification is based upon structural resemblances (homo- 

 logies) ; it follows the path of gradually increasing com- 

 plexity (differentiation) ; it is corroborated by the results 

 of embryology and palaeontology. 



