BRYOZOA. 97 
Introduction. ] 
Living Bryozoa are all inhabitants of water, and mainly of the sea, occurring in 
all zones and at varying depths, though seeming in general to prefer clear and shal- 
low water. With the single exception of the genus Loxosoma, they are composite 
animals, which by the combined efforts of the individual polypides built up colonies 
of greater or less extent, and of either a calcareous, corneous, or membranaceous 
composition, by means of repeated, continuous gemmation. These colonies, in both 
the living and fossil forms, present so great a variety of form and habit, that it is 
difficult if not impossible, to express their growth by any definite formula, Some- 
times they grow in plant-like tufts, composed of series of cells variously linked 
together ; sometimes they spread over shells and other foreign bodies, forming en- 
tire crusts of exquisite pattern, or delicately interwoven threads ; sometimes they 
rose into coral-like masses, branching stems, and narrow or broad fronds; at other 
times the cell-bearing branches formed most beautiful and regular open-meshed 
lacework. 
However diverse the external aspect of the combined product, the small builders 
themselves conform to a simple and quite definite type. Considered briefly, the 
polypide consists of an alimentary canal in which three distinct regions, an cesopha- 
gus, stomach, and intestine, are recognizable. This is enclosed ina sac, and bent 
upon itself so that its two extremities or openings approximate, one of them, the 
oral, being furnished with a number of slender, hollow, and ciliated tentacles, 
whose movement causes the food to be brought to the mouth. As a rule, the anal 
opening is situated without the ring of the tentacles. Generally the upper surface 
of the sac is flexible and capable of being invaginated by the action of retractor 
muscles attached to the alimentary canal, so that when the animal retreats into its 
cell the inverted portion forms a sheath around the tentacles. Heart and vascular 
system are wanting, but a nervous ganglion is present, and reproductive organs are 
developed in various positions within the cavity of the cell. The oya may be 
developed in a special receptacle (marsupium) attached to the zocecium, or in an 
inflation of the surface of the zoarium, sometimes called a gonocyst; in other cases a 
modified zocecium (gonecium) is set apart for reproductive functions. The general 
term owcium is applicable to all these structures. Many Bryozoa are provided with 
appendicular organs called avicularia and vibracula. The avicularia may be pedun- 
culate, and sway to and fro, or they may be immovably attached to the zocecium, 
The vibracula are flexible, bristle-like appendages, set in the excavated summit of a 
knob-like elevation or blunt spine. The acanthopores found so frequently among 
paleozoic Bryozoa, were probably the supports of similar structures. 
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