PRIMITIVE MULTICELLULAR ANIMALS 



Fig. 9.4. Mcsozoans. A 

 through E are selected stages 

 in the life cycle of Dicyemida, 

 occurring in the excretory 

 organs of cephalopod mol- 

 lusks. F through H are 

 sexually reproductive phases 

 of various Orthonectida. A, 

 immature dicyemid in a very 

 young squid. B, later stage, 

 containing masses of cells 

 developing from agametes 

 to produce nematogens. C, 

 nematogen. D, nematogen 

 in optical section, showing 

 the internal cells which give 

 rise to further generations of 

 nematogens, or to other stages 

 known as rhombogens and 

 infusorigens. E, ciliated in- 

 fusoriform larva which leaves 

 the host. Beyond this point 

 the life cycle is unknown. 

 F, fertilization in an ortho- 

 nectid. G, mature female, 

 and //, mature male. The 

 larval stages of these forms 

 are masses of cells (plasmodia) 

 occurring in a variety of in- 

 vertebrate hosts. (Redrawn, 

 after various authors, from 

 L. H. Hyman, The Inverle- 

 brales; Protozoa through Cte- 

 nophora, copyright 1940 by 

 McGraw-Hill Book Co., Inc., 

 printed by permission.) 



Female 



Male 



■Testis 



271 



