20 



LECTURE II. 



ON THE CLASSIFICATION OF ANIMALS. 



THE HYDEOZOA, ACTINOZOA, POLYZOA, BRACHIOPODA, ASGI- 

 DIOIDA, LAMELLIBRANCHIATA, BRANCHIOGASTEROPODA, 

 PULMOGASTEROPODA, PTEROPODA, AND CEPHALOPODA. 



In giving an account of the lowest forms of animal life in the 

 preceding Lecture, I have substituted for a definition of each 

 class, a description of the structure of some particular member 

 of that class, or of the organic features which are most obviously 

 characteristic of the class ; because, in hardly any of those groups 

 has the structure of many, and widely different, members 

 been thoroughly and exhaustively worked out. 



I entertain little doubt, however, that the main features of 

 the description of Spongilla might substantially be taken as a 

 definition of the Sjwngida, and those of the description of Para- 

 moecium, as a definition of the Infusoria. On the other hand, 

 we possess no such complete knowledge of the vital cycle of 

 any Gregarina or Rhizopod ; and neither description nor defini- 

 tion of the corresponding classes, of a thoroughly satisfactory 

 kind, is attainable. 



No such difficulties beset us in studying the next class, the 

 Hydkozoa (comprising the Hydroid polypes and the Medasse), 

 which may be defined with as much precision as any group in 

 the Animal Kingdom. 



All the Hydrozoa exhibit a definite histological structure, 

 their tissues primarily presenting that kind of organization 

 which has been called cellular. Again, the body always ex- 

 hibits a separation into at least two distinct layers of tissue — 



