T . ^ .. . BKYOZOA. 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 l^ss 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 

 lace work. 



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 oesopha- 

 gus, stomach, and intestine, are recognizable. This is enclosed in a 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 ova 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 (gonoecium) is set apart for reproductive functions. The general 

 term ooecium 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 zooecium. 

 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. 



