GENERAL SURVEY OF THE ANIMAL KINGDOM. 



General Classification, 



Protozoa — unicellular. 

 Rhizopods, Infusorians, and Gregarmids. 



Metazoa — multicellular. 



Non-Chordate, Invertebrate, Chordate, Vertebrate, or 

 or Backboneless. Backboned. 



INVERTEBRATE METAZOA. 



After the unicellular Protozoa, we begin with those multi- 

 cellular animals which are least removed from the gastrula- 

 type, with the Sponges (Porifera). Next we rank the 

 Stinging - animals (zoophytes, jellyfish, sea-anemones) or 

 Cgelenterata. because their simplest forms are not much 

 more than gastrulse. 



The first two classes of Metazoa — the Sponges and the 

 Stinging-animals — are contrasted in some important features 

 with all the higher forms. 



Sponges and Ccelenterata. 



Higher Animals (Ccelomata). 



There is no body-cavity. There is but 

 one cavity, that of the food-canal. 



There is no definite middle layer of cells 

 (mesoderm), but rather a middle 

 jelly (mesoglcea). 



The symmetry of the ^astrula is retained 

 in the adult, that is, the longitudinal 

 (oral-aboral) axis of the adult corres- 

 ponds to the long axis of the gas- 

 trula. 



There is a body-cavity or ccelome 

 between the food-canal and the walls 

 of the body. But this body-cavity 

 is often only incipient, or else almost 

 obliterated. 



There fs a distinct middle layer of cells 

 (mesoderm) between the_ external 

 ectoderm and the gut-lining endo- 

 derm. 



The longitudinal (head to tail, oral- 

 aboral) axis of the animal does not 

 correspond to the long axis of the 

 gastrula. 



Next to the Stinging-animals, I think that the simplest 

 " worms " should be ranked. To rank Echinoderms next 

 Coelenterates, as many text-books do, is confusing. Though 

 they seem to agree in symmetry, the transition is very 

 abrupt. The simplest " worms " (Turbellarians), however, 

 are not very far above the gastrula level. 



But beyond this beginning, we have little dear light. 

 There is no class of " worms," but an assemblage of classes 

 whose affinities are hard to determine. The word " worm " 



