2 THE INVERTEBRATA 



share , and the cuticle and segmentation which the Annelida and Arthro- 

 poda have in common — constitute one of these stocks. The other 

 comprises the Echinodermata and Chordata. Its members exhibit a 

 common mode of origin of the mesoderm, primitively as hollow 

 pouches, from the gut wall ; they possess, or give indications of, three 

 primary mesodermal segments ; the cleavage of their ova is entirely 

 different from that which is characteristic of the Annelida and 

 MoUusca ; and between the larvae of the lowest chordates (the En- 

 teropneusta) and those of certain echinoderms there is a remarkable 

 and detailed resemblance. 



The remaining phyla, smaller and less important, are difficult to 

 relate either to the foregoing groups or to one another. By the type of 

 cleavage of their ova and the possession of flame cells (p. 177), the 

 Platyhelminthes or flatworms seem to be related to the annelid stock. 

 Their lack of coelom is a difficulty in this respect. The structure of the 

 adults of the Rotifera and of the larva of the Polyzoa, which has the 

 character of a trochosphere, might link these groups to the same stock 

 Some other small phyla (Brachiopoda, Chaetognatha) have possibly 

 distant relationship to the echinoderm-chordate grouping. Others, 

 notably the Nematoda, are extremely isolated and enigmatical. 



In the great assemblage of triploblastic phyla, the backboned 

 animals, or Vertebrata properly so-called, stand as a branch of one 

 phylum, the Chordata. Yet their considerable numbers, the size, 

 high organization, and intelligent activity of their members, and the 

 fact that Man is one of them, give them an importance so great that 

 they have always been the subject of a distinct department of zoo- 

 logical study, and were at one time regarded as a primary branch of 

 the Animal Kingdom. That standing they have lost; but it is still 

 necessary for many purposes to treat them apart. 



The term *' Invertebrata " is retained to cover all the non-chordate 

 phyla and the chordates other than the Vertebrata. In that sense it 

 is used in this book. Only the Cephalochorda (Amphioxus), which, 

 though they are not vertebrates, have much in common with those 

 animals, are left aside as best studied with them. 



The limits of the several phyla are, with one or two exceptions, 

 agreed among zoologists. As much cannot be said for the lower 

 grades of the classification. Different views upon phylogeny, and 

 considerations of convenience, lead to many divergences as to the 

 extent and rank of the various divisions in the systems preferred by 

 different authorities; and even when there is agreement as to the 

 limits of a group different names may be applied to it. In no two 

 works will quite the same arrangement be found. This fact should be 

 borne in mind by the student in using the table of class^lfcation which 

 will be found as the Table of Contents at the beginning of this book. 



