GENERAL ZOOLOGY 



types inlermediate between highly organized colonial Protozoa and these 

 simple Metazoa, we must bridge the evolutionary gap with logical conjecture, 

 reasoning from the known facts to supply the missing information. 



The Phylum Mesozoa 



All Mesozoa are parasites within the bodies of other animals. They may be 

 defined as animals consisting of an outer syncytial or cellular layer, com- 

 monly ciliated, which encloses one or more cells giving rise to the gametes 

 and to another type of reproductive cells called agametes. The life cycle is 

 complicated and apparently includes asexual and sexual generations, which 

 alternate. The phvlum includes the single class Moruloidea, which contains 

 the order Dicyemida and the order Orthonectida. The members of this phylum 

 are the simplest of all the truly many-celled animals. This simplicity, how- 

 ever, is perhaps an outcome of degeneration, since all Mesozoa are parasitic 

 during the greater part of their life cycles. Parasites commonly show struc- 

 tural simplification, as compared with their free-living relatives, and the 

 Mesozoa may have degenerated greatly in the course of their evolution. Such 

 modification may have gone so far that it would be impossible to identify the 

 free-living type from which the Mesozoa have evolved, even if this type were 

 still in existence. Many zoologists regard the Mesozoa as greatly degenerated 

 flatworms, but there is no clear evidence for an evolutionary origin of this 

 sort. The Mesozoa constitute a small but well-defined group, important 

 because its members possess a simpler organization than that of any other 

 group of many-celled animals. They can, therefore, be taken at their face 

 value as the simplest metazoans of the present day, even though they may 

 have arisen from more complex ancestors and become simplified as a result 

 of their parasitic existence. 



Structure and Life Cycle. The dicyemids, as members of the Dicyemida 

 are called, occur as parasites in the excretory organs (nephridia) of squids and 

 octopi. They are small, elongate animals, a few millimeters in length, con- 

 sisting of very few cells, often a total of less than 25 (Fig. 9.4). An outer 

 layer of these cells, ciliated, encloses an inner axial cell or cells from which 

 the reproductive cells arise. The outer or somatic cells are differentiated into 

 a head region and a trunk region. The structure of a dicyemid is thus ex- 

 tremely simple, but the life cycle is complex and is not known completely for 

 any single species. Apparently, from the single axial cell, many cells are 

 formed which are called agametes because, although they are not ova, they 

 develop without fertilization. This is an instance, rare among Metazoa, in 

 which a single cell, which is a germinal cell but not a gamete, is capable of 

 producing the new generation. Development of the agametes gives rise to 

 numbers of asexually reproducing individuals, the nematogens. Eventually, 

 a new type of individual, the rhombogen, appears and produces still another 

 form, the infusorigen. Remaining within its parent rhombogen, the in- 



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