CHAPTER XIV 



MOLLUSCOIDA, TROCHELMINTHES, AND 

 CHAETOGNATHA 



These groups are rather conveniently considered in the same 

 chapter, because they are more or less isolated, small groups of the 

 unsegmented worm type. 



MOLLUSCOIDA 



This is the name of a group composed of two classes, as they are 

 treated here. It is usually considered a phylum name, but many 

 authors prefer to give each of the classes phylum rank. The justi- 

 fication of the latter plan may be questionable. 



Class Bryozoa (bri 6 zo'a — moss animals) includes a group of co- 

 lonial animals often called Polyzoa, which are similar to colonial 

 hydroids in their manner of growth and forms. It is true that their 

 structure distinguishes them very readily. Nearly all bryozoans 

 are marine, although there are a few fresh water forms. In ex- 

 ternal appearance they resemble certain of the corals and hydroids. 

 It was a long time after their existence was laiown that they were 

 separated from that group. The subclass Ectoprocta includes forms 

 in which the mouth is surrounded by tentacles and the anus is not 

 enclosed in this area. Bugula is an example of a treelike type of this 

 subclass. Another type is one that grows as an incrusting organism. 

 The second subclass Endoprocta is characterized by the circlet of oral 

 tentacles which also encloses the anus. 



Bugula 



Bugula is a common marine genus, the individuals of which are 

 associated in a treelike colony that lives attached to some object 

 in the water. These individuals are called zooids of which the soft 

 parts are known as polypide. They are within the primitive coelomic 

 cavity, the wall of which is the zooecium. The presence of retractor 

 muscles make it possible for each zooid to be withdrawn into the 

 vaselike part of the chitinous skeleton. There are some smaller 

 individuals whose shape is similar to that of a bird's head and 

 whose bodies are smaller than the other zooids. These are called 

 avicularia, and they are found on the surface of the colony. Their 



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