2O4 GEOLOGICAL BIOLOGY. 



and a central body cavity common to digestion and circula- 

 tion (gastro-vascular space). 



" Echinodermata. Radiating animals, for the most part 

 of pentamerous arrangement ; with calcareous dermal skele- 

 ton, often bearing spines ; with separate alimentary and vas- 

 cular systems ; and with nervous system and ambulacral feet. 



11 Vermes. Bilateral animals with unsegmented or uni- 

 formly (homonomous) segmented body, without jointed ap- 

 pendages (limbs), with paired excretory canals sometimes 

 called water-vascular system. 



" Arthropoda. Bilateral animals with heteronomously- 

 segmented bodies and jointed appendages, with brain and 

 ventral chain of ganglia. 



" Molluscoidea. Bilateral, unsegmented animals with cili- 

 ated circlet of tentacles or spirally rolled buccal arms ; either 

 polyp-like and provided with a hard shell-case, or mussel-like 

 with a bivalve shell, the valves being anterior and posterior; 

 with one or more ganglia connected together by a perioeso- 

 phageal ring. 



" Mollusca. Bilateral animals with soft, unsegmented 

 body, without a skeleton serving for purposes of locomotion ; 

 usually enclosed in a single or bivalve shell, which is ex- 

 creted by a fold of the skin (mantle) ; with brain, pedal-gan- 

 glion, and mantle-ganglion. 



" Tunicata. Bilateral unsegmented animals with sac- 

 shaped or barrel-shaped bodies, and a large mantle cavity per- 

 forated by two openings ; simple nervous ganglion, heart, and 

 gills. 



" Vertebrata. Bilateral animals with an internal cartilagi- 

 nous or osseous segmented skeleton (vertebral column) which 

 gives off dorsal processes (the neutral arches) to surround a 

 cavity for the reception of the spinal cord and brain ; and 

 ventral processes (the ribs) which bound a cavity for the re- 

 ception of the vegetative organs ; never with more than two 

 pairs of limbs." 



Zittel adopts the older Claus classification, in which the 

 fifth branch, Mollusca, includes Molluscoidea, Mollusca, and 

 Tunicata divisions which are given higher rank in the newer 

 classification. 



