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 
on 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 man}' 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 bi'yozoa 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 
