74 THE MOST PRIMITIVE METAZOA 



(2) They have a solid blastula. 



(3) There is a simple adult form; there are no proglottides, 

 no suckers, no thick cuticle, no nervous system, no flame 

 cells, no complex gonadial system. 



(4) They have cilia and a few reproductive cells as their 

 specialisations. 



(5) The cilia of the trematode miracidium larva are soon lost ; 

 those of the Mesozoa last throughout the life of the animal 

 (not in the Orthonectids). 



(6) There is no cell in the miracidium comparable to the 

 internal nematogen cells of the adult Dicyemids. 



Resemblances between the Mesozoa and the digenetic 

 trematodes 



(1) They are internal parasites. 



(2) They have a complex life cycle. 



(3) Both the trematodes and the Mesozoa show polyembryony. 



(4) The trematode miracidium larva and the Orthonectid 

 ciliated larva have the following similarities. 



(a) The larva results from similar unequal cleavage of the 

 fertilised ovum. 



(b) The larva is bilaterally symmetrical. 



(c) The larva has a fixed number of cells. 



(d) The larva is ciliated. 



(e) The larva does not feed. 



(/) On arrival in the host, the somatic cells degenerate 



and the generative cells develop. 

 (g) The larva is the distributive phase between one host 



and the next. 



(5) The adult male Orthonectid has a reproductive duct. 



Details of the structure and life history of the Mesozoa are given 

 in Figs. 25-28. Our knowledge of the life cycle of the Ortho- 

 nectids is fairly complete but that of the Dicyemids is not. Thus 

 we do not know if they have a second host, and, if so, the 

 morphology of the parasite in this host. A suggested life cycle is 

 shown in Fig. 28 and this cycle is more complex than that 

 described for the Orthonectids (McConnaughey 1951). 



