GENERAL PRINCIPLES 



19 



Worms, and Arthropods in the entire absence of body segmenta- 

 tion. Absence of distinct head and foot, the lower development 

 of the nervous system, and usual lack 

 of locomotive power distinguish them 

 from Mollusks. A highly characteristic 

 feature of the Molluscoids is a sort of 

 collar, or pair of arms (often containing 

 carbonate of lime) of varying shapes 

 and bearing fringe-like tentacles around 

 the mouth. 1. Bryozoans form tiny 

 moss-like tufts which nearly always 

 colonize and suggest the Anthozoans in 

 outward appearance, though they are 

 much more highly organized. With few 

 exceptions the Bryozoans secrete calca- 

 reous Coral-like skeletons. 2. Brachi- 

 opods are characterized by two distinct, 

 external shells (valves) which contain 

 the soft portion of the animal, and also 

 a pair of long, spirally coiled, fringed arms. In fossil form the 

 Brachiopods are most readily distinguished from certain Mollusks 

 (Pelecypods), which are also bivalves, by the bilateral symmetry of 



Fig. 10 

 Brachiopod shells (fossil 

 forms). The lower one 

 shows the internal spiral 

 arms. 



Fig. 11 

 A modern Pelecypod. A, side view; B, end view. (After 

 Howes, from Schuchert's "Historical Geology," permission 

 of John Wiley and Sons.) 



the shells. That is, a plane of symmetry may be passed through the 

 valves at right angles to the hinge line. Bryozoans and Brachi- 

 opods are both very abundant as fossils, especially in the older rocks. 



