GENERAL ZOOLOGY 



Fig. 9.5. Types of sponges. ^, Calcarea: 

 Scypha, three living individuals. B, 

 Hexactinellida: portion of the skeleton 

 of a glass sponge, Eupiectella. C, Mon- 

 axonida; Microciona. {A and C, photo- 

 graphs by George Lower; fi, photograph 

 by Bassett Maguire, Jr.) 



fu.sorie^en develops male and female gametes. These unite in syngamy, and 

 the divisions of the zygotes thus formed give rise to numerous infusoriform 

 larvae. The larvae are liberated from the body of the parent (and grand- 

 parent) and leave the host, but their further history is completely mysterious. 

 They survive only brief periods in sea water and are apparently incapable of 

 infecting squids and octopi. It thus appears that an unknown intermediate 

 host must be involved in the cycle but, despite the efforts of many com- 

 petent investigators, further information is unavailable. 



In the Orthonectida, the life cycle also includes parasitic asexual phases; 

 these are more or less structureless, multinucleate masses of protoplasm in- 

 habiting a variety of invertebrate hosts. In addition, free-living males and 

 females are known which produce sperm and egg cells. These cells are re- 

 leased into the sea and unite in syngamy to form zygotes, which develop into 

 ciliated larvae resembling those of dicyemids. The larvae are infective for 

 the invertebrate hosts, and, having gained entry, transform into the asexual 

 form (Fig. 9.4). 



The Body Plan and Life Cycle. To generalize, the mesozoan body 

 consists of a small number of body cells, surrounding a parent germ cell 

 capable of producing many agametes. The agametes are single cells, each of 

 which can reproduce a new individual. In .some and perhaps all species 

 gametes are eventually produced, so that there is an irregular alternation 

 of sexual and asexual generations. Although the mesozoan is hardly more 

 complicated than some colonial protozoans, it is clearly a metazoan animal. 

 Even if its simplicitv has resulted secondarily from a parasitic mode of life, 

 it is an example of a very lowly type of metazoan. If its simplicity is really 

 primitive, the mesozoan type is very important because it suggests a possible 

 step in the evolution of many-celled from single-celled animals. 



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