340 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



The bryozoa are perhaps best known to-day from the paperlike 

 fronds called " sea mats " and the mosslike structures tossed upon our 

 seacoasts. These are not plants as was long supposed 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 placed 

 in a halfway group termed "zoophytes," partly animal and partly 

 plant. The coral-like appearance of the calcareous bryozoa gave 

 origin to another term " corallines." When it was discovered, how- 

 ever, that each individual cell of the composite colony contained an 

 animal with a complete alimentary canal totally unlike the corals or 

 any other group with which they had been compared, the name 

 " Bryozoa " was definitely introduced for them as a new group of 

 animals by C. G. Ehrenberg in Germany in, 1831. Another term, 

 " Polyzoa," applied by J. B. Thompson in Ireland in 1830, was not 

 so precisely defined, and a long controversy arose concerning the 

 two rival terms. This has been settled by a curious division of opin- 

 ion, namely, the term " Polyzoa " is preferred by most English natu- 

 ralists, but all of the continental and American authors employ the 

 designation " Bryozoa." 



GENERAL CHARACTERS. 



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 ancestrida. 

 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 sur- 

 rounded by the lophophore, bearing a crown of hollow, slender, 

 ciliated tentacles arranged in a circle or crescent by which micro- 

 scopic organisms such as diatoms are gathered for food. 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 zotwium; it presents a great variety of form and structure, 

 although the form is quite constant in individual species. Very fre- 

 quently 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 ir- 

 regular masses, often of considerable size, may result. Again, the 

 zoaria may arise in fronds or branching stems, and at other times 



