MISCELLANEOUS MINOR PHYLA 



169 



length. They Hve in the mud or sand on the 

 bottom of either deep or shallow water. The 

 body consists of a series of 13 or 14 rings, 

 two of which form the head, which is en- 

 circled by spines and has a short retractile 

 proboscis. There are two excretory' organs, 

 each consisting of a flame cell connected to 

 a flagellated ciliated duct opening dorsally 

 on ring 9. Echinoderes dujardini (Fig. 89) 

 is reddish in color; it lives in mud and less 

 often among algae in the north Atlantic 

 Ocean. 



PHYLUM MESOZOA 



These are small slender animals, with the 

 simplest structures of any metazoan. They 

 are parasites; and their simplicity may be 

 partly the result of modifications due to a 

 parasitic existence. They live in the internal 

 spaces and tissues of squids, flatworms, star- 

 fishes, annelids, and other invertebrates. The 

 body consists of an outer layer of cells en- 

 closing one or more reproductive cells. The 

 life cycle is complicated by an alternation 

 of sexual and asexual generations. Dicyema 

 lives in the nephridia of the octopus, and 

 Rhopalura (Fig. 89) parasitizes the gonads 

 of the brittle star. 



The Mesozoa resemble some colonial 

 protozoans in that they have external cilia, 

 digestion which occurs in external cells, 

 and special reproductive cells as in Volvox. 

 The Mesozoa are unlike the typical meta- 



zoans in that their two-cell layers are not 

 comparable with the ectoderm and en- 

 doderm of typical metazoan animals, and 

 they have no internal digestive tract. The 

 name Mesozoa implies that these organisms 

 are intermediate between the Protozoa and 

 Metazoa, which, indeed, they may actually 

 be. Either they are intermediate between 

 unicellular and multicellular animals or else 

 degenerate forms. Some of the best authori- 

 ties believe that their characters are chiefly 

 primitive and not the result of parasitic 

 degeneration. 



SELECTED COLLATERAL 

 READINGS 



Bassler, R.S. "The Bryozoa, or Moss Animals." 

 Smithsonian Inst. Ann. Rept., 1920. 



Hyman, L.H. The Invertebrates: Acanthoce- 

 phala, Aschelminthes, and Entoprocta. Mc- 

 Graw-Hill, New York, 1951. 



Johnson, M.E., and Snook, H.J. Seashore Ani- 

 mals of the Pacific Coast. Macmillan, New 

 York, 1927. 



MacGinitie, G.E., and MacGinitie, N. Natural 

 History of Marine Animals. McGraw-Hill, 

 New York, 1949. 



Michael, E.L. "Classification and Vertical Dis- 

 tribution of the Chaetognatha of the San 

 Diego Region." Univ. Calif. Pub. 7.ool., 

 1911. 



Ward, H.B., and Whipple^ G.C. Freshwater 

 Biology. Wiley, New York, 1918. (New 

 edition in press.) 



