NORTH AMERICAN EARLY TERTIARY BRYOZOA. 39 



GENERAL DESCRIPTION OF THE BRYOZOA. 



The paper-like fronds called " sea mats " and the moss-like structures tossed up 

 311 our sea coasts to-day are not plants as they were long supposed to be, but are 

 animal colonies consisting of a great number of small cells opening side by side. 

 Before their true nature was learned, these organisms were termed zoophytes or 

 corallines, but when it was discovered that each individual cell of the composite 

 colony contained an animal with a complete alimentary canal, the name Bryozoa, or 

 moss-like animal, was coined for them. Another term, Polyzoa, was introduced for 

 the same group and is preferred by many English naturalists, but all of the conti- 

 nental and American authors employ the designation Bryozoa. 



In spite of the great abundance of bryozoa in the recent seas and their very 

 frequent occurrence as fossils, knowledge of their structure is unfortunately usually 

 limited to the special student. For this reason the following remarks, devoid of 

 scientific terms, so far as possible, have been introduced. 



The bryozoa are small, composite, usually marine, animals arising from a free- 

 swimming larva which becomes attached to some foreign object and then develops 

 into the primary individual or ancestrula. By repeated budding from the ancestrula, 

 colonies of various shapes and sometimes considerable size arise. Each individual 

 animal or zooid is composed of a double-walled membranaceous or calcareous sac, 

 the zooecium, within which is the visceral mass, the polypide, consisting of a freely 

 suspended alimentary canal U shaped so that the mouth and anus open close to each 

 other. The mouth is surrounded by the lophophore bearing a crown of hollow, 

 slender, ciliated tentacles arranged in a circle or crescent. Both sexes are usually 

 combined in the same zooid. It is a curious fact that the same zooecium may be 

 inhabited at different times by different polypides. 



The colony which the individual zooids form is known technically as the 

 zoarium; it presents a great variety of form and structure, although the form is 

 quite constant in individual species. Very frequently the zoaria grow over shells, 

 stones, or other bodies, forming delicate incrustations of exquisite patterns. By 

 the superposition of many such incrustations, hemispherical, globular, nodular, or 

 irregular masses often of considerable size may result. Again the zoaria may 

 arise in fronds or branching stems, and at other times they form open-meshed 

 lacework of the most regular and beautiful patterns. Most bryozoa are attached 

 either basally or by the greater part of their surface to extraneous objects, or are 

 moored to the bottom by root-like appendages. In many forms the zoarium is 

 regularly jointed to give greater mobility. 



The individual zooids of the zoarium conform to a simple and definite type 

 of structure throughout the class. The soft parts of the animal consist of an 

 alimentary canal with three distinct regions discernible, esophagus, stomach, and 

 intestine. The alimentary canal is inclosed in a sac and bent upon itself so that 

 the two extremities are close to each other. The mouth or oral opening is either 

 entirely or partially surrounded by a row of slender, hollow, ciliated tentacles 

 which serve for respiration and for sweeping food toward the mouth. The two 



