CHAPTER I 



INTRODUCTION 



The Invertebrata have long since ceased to constitute one of the 

 primary divisions in the scientific classification of the Animal King- 

 dom. Their name is now no more than a convenience for designating 

 a group of phyla with which it is often necessary to deal as a whole. 

 The primary lines of real cleavage in the Animal Kingdom divide it, 

 not into Vertebrata and Invertebrata, but into three unequal sec- 

 tions, the Protozoa, Parazoa and Metazoa, which are ranked in the 

 following chapters as subkingdoms. 



Between the Protozoa, which are without cellular differentiation 

 and contain a large group of photosynthetic members, and the Meta- 

 zoa, in which such differentiation is always strongly marked and 

 photosynthesis is absent, there is a gulf which is in fact far deeper 

 than that which sunders the Protozoa from the lower plants. The view, 

 indeed, has been put forward that these two components of the 

 Animal Kingdom are not, as is usually held, directly related to one 

 another, but arose, with the Plants, as entirely distinct branches of an 

 ancestral stock of living beings. The Parazoa or sponges — unique 

 among many-celled organisms in possessing collared flagellate cells — 

 are probably derived from the Protozoa by an origin distinct from 

 that by which the latter group gave rise (if they did so indeed) to the 

 Metazoa. 



Within the Metazoa, the most significant difference is that which 

 exists between the Coelenterata or Diploblastica and the triploblastic 

 phyla which constitute the rest of the subkingdom. The Coelenterata, 

 which typically start life as a simple, two-layered, ciliate larva, the 

 planula, either retain throughout life the two-layered condition, or 

 depart from it only by the immigration, late in development, of cells 

 from the two primary layers (ectoderm and endoderm, p. 128) into 

 the space (blastocoele) between those layers. The triploblastic animals 

 always possess a true third layer (mesoderm) which is early developed 

 and forms important organs. They are the great majority of animals, 

 and compose a number of phyla. 



The brigading of these phyla is a difficult task — one, indeed, which 

 it is at present impossible to effect completely. Two main stocks, 

 however, stand out fairly clearly. The Annelida, Arthropoda and 

 Mollusca — by the plan of their central nervous system, the mode and 

 position of origin of their mesoderm, the types of cleavage of the ovum 

 (p. 281) and of larva (the ^roc^oj'/)^er^) which the Annelida and Mollusca 



