240 GEOLOGICAL BIOLOGY. 



(Vermes), and treats of Brachiopoda as a distinct 

 class, but allies it on certain accounts with the 

 Vermes.) 

 B. Mollusca (proper). 



Class I. Laincllibranchiata. 



2. Gastropoda. 



3. Cephalopoda. 



The embryologists make greater point of resemblances 

 observed in the early stages of development, and hence the 

 distribution made of Bryozoa and Brachiopoda next the 

 Worms, and Tunicata next to Vertebrates; but when the 

 mature animals are studied and compared the Brachiopoda 

 are found to possess structures closely resembling in impor- 

 tant features the Mollusca proper 



The embryological resemblance of Bryozoa and Brachi- 

 opoda to worms is lost when the adult stage is reached. 

 Hence, for the geologist particularly, the association of the 

 two is not suggested by any apparent similarity of characters. 



Points of View of the Embryologist and of the Morphologist. — 

 In studying the philosophy of natural history it is interesting 

 to note this difference in point of view between the strict 

 embryologist and the pure morphologist. They compare 

 animals on a different basis, and therefore there results in 

 some cases a different classification. 



The embryologist classifies animals primarily on the theory 

 of phylogenetic relationship; the student of adult morphology 

 classifies them according to the nature and extent of differen- 

 tiation attained in the adult. Here, too, the two examples 

 selected will illustrate the differences. The two modes of 

 classification differ much as the classification of houses might, 

 viz. by considering them, either, according to the styles or 

 schools of architecture, as Norman, Roman, Queen Anne, 

 etc., on the one hand, or according to the materials of con- 

 struction — brick, stone, or wood^ on the other. 



The line of descent, through which any particular organ- 

 ism has come to be what it is, is all-important if it can be 

 discovered by the study of embryology; but of no less impor- 

 tance is it to distinguish the different and similar structures 

 which have been developed for the accomplishment of the 



