CHAPTER VIII 

 PHYLUM TROCHELMINTHES 



Many of the higher invertebrates in their development pass 

 through a larval stage known as the trochophore. In most 

 instances where a trochophore is involved, it later by meta- 

 morphosis gives rise to an adult animal which in organization 

 is fundamentally different from the simple larva. There are, 

 however, a few organisms which in their adult state are not 

 essentially different from the trochophore type of organization. 

 The most characteristic of these is the group of the Rotifera. 

 In addition to the rotifers, there is a small group of minute 

 fresh-water organisms known as the Gastrotricha which are in 

 some respects similar to the Rotifera. These two groups are 

 united to form a phylum to which the name Trochelminthes is 

 frequently applied. 



By some, representatives of this phylum are thought to repre- 

 sent precociously mature larvae, or an instance of what might 

 be termed phylopaedogenesis. 



Class Rotifera 



The Rotifera, or wheel animalcules, are microscopic animals 

 which in fundamental structure closely resemble the trochophore. 

 In size and superficial appearance, they might be mistaken for 

 Protozoa and were so considered by many of the early workers. 

 Close observation reveals in them miniature organ systems and 

 demonstrates their true metazoan natures. Most rotifers live in 

 fresh water though a few dwell in seas. No body of water is too 

 large or too small for them, for tiny temporary pools frequently 

 support a varied fauna of these minute organisms. Many are 

 capable of withstanding desiccation and one type of eggs is highly 

 resistant. These two facts go far toward explaining the prac- 

 tically cosmopolitan distribution of many species, for the dried 

 individuals or eggs could be transported great distances or 

 might even be carried by the winds. Some genera are commonly 

 represented in the fresh-water plankton, others are character- 



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