THE ANIMAL KINGDOM 13 



single evolutionary series, but we have no certain 

 knowledge as to whence the Molluscan type has come 

 or with what other phylum it has any close relation- 

 ship. At some very remote time we may suppose 

 that the Mollusca had a common origin with the 

 Appendiculata, since a larval form, the Trochosphere, 

 is common to certain members of both phyla. The 

 Mollusca are also coelomate animals like the Appen- 

 diculata, but they possess no trace of metameric 

 segmentation, and there is nothing to indicate 

 whether they branched off from the common stock 

 of the Appendiculata before segmentation had been 

 acquired, or whether they have subsequently lost this 

 characteristic. 



We may omit certain small and little known 

 phyla, often containing only a few organisms of 

 altogether obscure origin, and pass on to the last 

 remaining important phylum, perhaps the most im- 

 portant of all, as it comprises all the highest forms 

 of animal life, the Chordata. The Chordate type, 

 such as is exhibited by any typical Vertebrate 

 animal, for instance a Fish, a Reptile or a Mammal, 

 possesses a number of distinctive features. Like the 

 Appendiculata they are metamerically segmented, 

 but this segmentation, although it is apparent in 

 the embryo, becomes obscured in the adult, which 

 palpably exhibits its segmentation only in the 

 vertebral column or backbone and in the spinal 



