740 



ZOOLOGICAL SOCIETY BULLETIN. 



TUBULIPOKA ATI ANTICA AND OVICELl.. ENLARGED 

 From a drawing by Mr. H. J. Shannon. 



BRYOZOA OR MOSS-ANIMALS. 



THE Bryozoa or moss-aniiuals, so named 

 from tlie fact that, certain of them grow in 

 moss-like clusters, are common members of 

 our marine fauna and yet they are known to but 

 very few persons. They are known also as 

 Polyzoa from the fact that they are colonial in 

 habit. The individuals are minute, rarely as 

 large as the head of a pin and usually much 

 smaller, yet their power of asexual reproduction 

 by budding is so highly developed that they 

 often form considerable masses. 



In habit of growth they are extremely varied. 

 Perhaps the commonest method is that of en- 

 crusting stones, shells, sea-weed, etc., with a 

 layer formed of many zooe- 

 cia or individual animals. 

 They usually remain closely 

 attached to each other so as 

 to form a continuous crust, 

 the walls of which are com- 

 monly impregnated with 

 lime, giving great strength 

 and rigidity. Occasionally, 

 however, tlie walls may be 

 merely' horny or even gela- 

 tinous, and in a few cases 

 the walls are covered with 

 earthy matter which renders 

 them inconspicuous against 

 the sea bottom. In the case 

 of many of these encrusting 

 species one layer may form 

 upon the top of another, the 

 lower one dying and leaving 



only its lime .skeleton, and 

 this ])rocess may go on im- 

 til a considerable mass is 

 formed, consisting of many 

 dead layers and a superficial 

 living one. Such masses may 

 be mereh' flat layers or they 

 may form nodules by com- 

 (iletely enclosing shells or 

 ))ebbles, or less conmionly, 

 they may rise free from the 

 encrusting base in frill-like, 

 tree-like or cup-like forms, 

 not infrequently of great 

 beauty and .symmetry. 



In another common meth- 

 od of growth the colony is 

 erect from the beginning. 

 In such S]5ecies tlie first in- 

 dividual of the colony 

 (known as the "ancestrula") 

 attaches itself and the sub- 

 sequent ones are developed from it in such a 

 manner as to form a branching, tree-like colony 

 often very complex. Usually these forms are 

 flexible, either by means of definite joints situ- 

 ated at certain intervals (nodes), or else the 

 whole colony is uniformly flexible by reason of 

 the pliability of the walls of the zooecia or 

 tlieir looseness of articulation. 



While all the Bryosoa reproduce asexually by 

 budding, they also produce eggs which develop 

 into free-swimming ciliated larvae. These are 

 so different from the adult individuals that they 

 were not at first connected with the Bryozoa, 

 but were described separately. The best known 

 of these larval forms is the larva of Membrani- 



SMITTIA NITIDA, AN ENCRUSTING SPECIES. 

 Natural size. Photo by R. C. Osburn. 



