204 GEOLOGICAL BIOLOGY. 



and a central body cavity common to digestion and circula- 

 tion (gastro-vascular space). 



"■ EcJiijiodermata. — Radiating anit aals, 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. 



" 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. 



'■'■ ArtJiropoda. — Bilateral animals with heteronomously- 

 segmented bodies and jointed appendages, with brain and 

 ventral chain of ganglia. 



^^ Molhiscoidea. — 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. 



^^ Mollusea. — 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. 



" Tiinicata. — 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. 



