18 

 Phyla of Lesser Significance ' 



There are a number of animal groups that are considered as separate 

 phyla ; yet, because they are little known, of little economic importance, 

 or have no great biological significance, they do not deserve a thorough 

 study in a survey course. However, in order to make the survey of the 

 animal kingdom complete, they are listed in this chapter together with 

 some of their most important characteristics. 



Phylum — Ctenophora 



This phylum includes the comb jellies, or the sea walnuts. The first 

 common name is given because they bear eight rows of small ciliated 

 bodies which closely resemble combs and their bodies are jelly-like in 

 consistency. The shape of some of them has given them the name of sea 

 walnuts. They are marine forms which float and swim near the shore, 

 and great numbers are sometimes cast up on the beach by the breakers. 



The Ctenophora are sometimes placed in the Coelenterata because 

 their body organization is quite similar to the medusa type in this phy- 

 lum. However, there are certain important morphological differences 

 that put them in a separate group. The jelly-like mesoglea contains 

 long muscle fiber cells which seem to be forerunners of a mesoderm and 

 the triploblastic body plan as found in the Platyhelminthes. The Cte- 

 nophora have a type of symmetry which is called biradial ; it is radial 

 in nature, but the parts are equally arranged on two sides so that it bears 

 some relation to the bilateral type of symmetry. 



Phylum — Rotifera 



This phylum includes a number of interesting microscopic forms that 

 are frequently found in fresh-water cultures along with protozoa. The 

 phylum name means to bear a wheel, and rotifera are commonly called 

 wheel animals because many of them have cilia on disc-shaped projec- 

 tions from the anterior end of the body which beat in such a way that the 



1 This chapter may be omitted without interrupting the continuity of the text if 

 the instructor so desires. 



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