CHAPTER XII 



The Moss Animals 



{Phylum Bryozoa or Edoprocfa) 



A, 



-NYONE curious about animal life in water is 

 almost sure to meet moss animals (bryozoans) in 

 many guises. A piece of seaweed is cast upon the 

 beach: over some of the floats and leaflike areas is 

 a limy coating with a pattern of minute pores. This 

 encrustation is a "lace coral," the work of one type 

 of bryozoan (Plate 33 ). Or the spire of a whelk shell 

 is rough with a different limy coating; the colony of 

 another moss animal. 



The nautically minded often meet bryozoans. A 

 skiff, pulled ashore after a summer at its mooring in 

 salt water, must be examined underneath for bar- 

 nacles and other fouling organisms. Some of the 

 shrubby and fuzzy growths are almost certain to be 

 bryozoans. Even in fresh water toward autumn, the 

 piers of a boat dock may develop enormous masses 

 of gelatinous material patterned in a mosaic of 

 brown markings over the surface. This too is a co- 

 lonial moss animal, not the egg mass of a giant frog. 



Moss animals are all colony builders, and never 

 live alone. Each individual is of microscopic dimen- 

 sions, seldom more than ^^',4 inch in length. It lives 

 a few weeks attached to the walls of a chamber 

 formed of its own secretion while capturing still 

 more minute particles of food in a current of water 

 created by cilia on its many tentacles. The presence 

 of cilia on the tentacles distinguishes a moss animal 

 from any coelenterate hydroid. 



Bryozoans have a U-shaped digestive tract in which 

 the mouth is centrally placed in a ring or horseshoe- 



shaped group of tentacles and the anus lies near the 

 mouth but is not encircled by the tentacles. The anus 

 is exposed when the tentacles are fully extended from 

 the chamber housing the animal. Otherwise the body 

 of each individual is astonishingly simple. It contains 

 no respiratory, circulatory, or excretory system. Nor 

 do the reproductive organs open to the outside by 

 organized passageways. 



The brevity of life span for individual moss ani- 

 mals could be suspected from examining a healthy 

 colony with a hand lens. Each community begins as 

 a sexually produced single individual maturing from 

 a newly attached juvenile which has just gone through 

 marked changes from the embryonic swimming stage. 

 The first individual produces asexual buds, each of 

 which adds another chamber and another zooid — 

 and more buds. Consequently the periphery of a 

 growing incrusted colony or the tips of the branches 

 of a feathery clump of bryozoans is always the 

 youngest part. 



Back from the edge or the tips, the hand lens usu- 

 ally reveals chambers empty except for minute brown 

 lumps, the "brown bodies"" which remain from a de- 

 generated individual zooid. Still older parts of the 

 colony are likely to be inhabited again by feeding in- 

 dividuals, for into the chamber of a dead zooid the 

 colonial cross-connecting strands send a new bud to 

 provide a replacement. Often the first meal of the new 

 zooid is the brown body representing its predecessor. 



No one is sure why each zooid dies so young. Pos- 



150] 



