574 THE ANATOMY OF INVERTEBRATED ANIMALS. 



The Physemaria, the Porifera, the Hydrozoa, the Coral- 

 ligena, and the Ctenophora, are obviously modifications of 

 the same fundamental plan. I think it is convenient to re- 

 tain the well-established name of Gcelenterata for the last 

 three orders, which are much more closely related to one 

 another than to the other two. Haeckel's proposal to apply 

 the old name of Zoophyta to the whole division appears to 

 me to be well worthy of adoption. The inconvenience of 

 using a term the connotations of which have varied some- 

 what widely since it was first invented, is probably less than 

 that which would attend the invention of a new name. 



The Jfonera, Foraminifera, Heliozoa, Hadiolaria y Pro- 

 toplasta, Gregarinida, Oatallacta, and Infusoria (Opali- 

 ?iina, Ciliata, Tentaculifera, Plagellata), again, are so close- 

 ly united together that the difficulty is to distinguish the 

 less differentiated forms of each from one another. They 

 constitute the division of the Protozoa, the common charac- 

 ters of which have been given in Chapter II. 



If there were no invertebrated animals besides those in- 

 cluded under these four divisions of Arthropoda, Mollusca, 

 Zoophyta, and Protozoa, the task of classification would be 

 very easv, and each of the higher divisions would be sharply 

 defined from the others. But a vast residuum remains to be 

 considered ; and it is with the attempt to arrange these resid- 

 ual orders into higher groups that the difficulties of the Tax- 

 onomist commence. 



The Polychoeta and the Oligochceta, the Hlrudinea and 

 the Gephyrea, resemble one another generally in the seg- 

 mentation of the body, indicated at least by the serially mul- 

 tigangliate nervous centres ; 1 in the presence of cilia and of 

 segmental organs ; and in the nature of the larva?, which are 

 set free when their embryos are hatched in an early stage of 

 development. And, although no one of these characters is 

 of universal occurrence (cilia, for example, being absent in 

 most adult Hirudinea), yet they are found in such association 

 that the accepted arrangement of these four groups (to which, 

 though not without some hesitation, I add the Myzostomata) 

 into the division of the Annelida is undoubtedly very con- 

 venient. 



The Trematoda, the Turbellaria, and the JRotifera, form 



1 This character is wanting in most Gepliyrea, which, as I have remarked 

 at p. 218, incline in many respects toward the next division, and especially 

 toward the Rotifera and Nematorhyncha. 



