CHAPTER IV 



Primitive Animal Forms 

 Branched and Segmented Animals 



/^\WING to their aptitude for changing their form and 

 ^S moving about, those members of the animal kingdom 

 reduced to a single structural element or plastid, acquired a 

 variety of forms infinitely greater in number than that which 

 we find at the corresponding stage in the Vegetable Kingdom. 

 Furthermore, these plastids, having once become associated, 

 modified each other reciprocally and became mutually 

 dependent much more quickly than the vegetable plastids. 

 Hence, we do not find in the Animal Kingdom, alongside of 

 unicellular beings which constitute the large group of Protozoa 

 corresponding to the simplest structural type, creatures of larger 

 size possessing the homogeneous structure encountered in the 

 Algae and the higher Fungi. We pass suddenly from the 

 Protozoa to organisms already complicated ; the Protozoa, 

 however, possess an infinite variety of forms and abound every- 

 where. They are divided into three large groups, the Rhizopoda, 

 Infusoria, and Sporozoa. 



The body substance of the first has a consistency so 

 nearly akin to that of water that the surface of the mass 

 responds to the least attraction ; it is constantly being 

 flocculated, fringed, or lobed, and the temporary amoebic 

 protrusions that emerge from its mass are called pseudopodia, 

 that is, false feet. These pseudopodia, if they are ex- 

 tensively ramified, may have the delicate ramifications 

 fused together. Thus, they become surrounded by a kind 

 of living network. Two classes of these reticulated Rhizo- 

 poda have played a great part in all Geological Periods, 

 and are still abundant in all our seas ; these are the Radiolaria 

 with a skeleton, which is often silicious, and the Foraminifera, 

 with a shell that is generally calcareous. The first float on the 

 water, and their skeletal debris is found as far back as the 

 Algonkian deposits ; the second group live nearer or actually 



