OTHER UNSEGMENTED WORMS 



189 



by an anus situated near the mouth and either within or without the 

 lophophore. There are neither circulatory nor excretory organs. The 

 nervous system consists of a central ganglion between the mouth and 

 anus. Reproduction is both sexual and asexual, the latter taking place 

 by budding. These animals are either monecious or diecious. The eggs 

 are fertilized in the coelom and develop in a modified portion of the body 

 cavity called an ooecium which serves as a brood pouch. The larva is 

 in many respects like a trochophore. 



Certain species produce individuals of a peculiar type called avicularia. 

 These are highly modified forms possessing a pair of strong jaws, which 

 probably are used in defense. 



A B 



Fig. 98. Fig. 99. 



Fig. 98. — Stato blasts of two genera of fresh-water bryozoans. A, Plumatella. B, 

 Cristatella. Much magnified. 



Fig. 99. — Two specimens of Tcrebratella transversa (Sowerby), a brachiopod common 

 on the Pacific Coast. Natural size. 



The fresh-water bryozoans (Fig. 97) form mosslike colonies attached 

 to plants, sticks, or stones, usually near the surface of quiet water, 

 though they have been found in Swiss lakes at a depth of over 40 fathoms. 

 Some have the power of movement, the whole colony very slowly crawling 

 along on its base. These fresh-water forms have developed a type of 

 winter egg known as a statoblast (Fig. 98), which is produced in the fall, 

 inclosed in a chitinous shell, and may either fall to the bottom of the 

 body of water in which it is or float. Freezing does not interfere with 

 its development in the spring but rather seems to stimulate it. 



An enormous number of fossil forms have been described and some 

 of these are very similar to species now living. 



221. Phylum Brachiopoda. — The animals included in Brachiopoda 

 (brak i op' o da; G., hrachion, arm, and podos, foot) resemble certain 

 mollusks in that they possess a bivalve calcareous shell (Fig. 99). For 

 that reason they have in the past been generally considered as belonging 

 to the field of conchology, the science which deals with the mollusks. 

 They have also been frequently grouped with the Bryozoa in a phylum 

 called Molluscoidea. The brachiopods, however, differ from the bivalve 



