CHAPTER IV. 



AND ORTHONECTID^. 



DICYEMID^:. 



THE structure and development of these remarkable para- 

 sites in the renal organs of the Cephalopoda have recently been 

 greatly elucidated by the researches of E. van Beneden ; and 

 although a male element has not been discovered, yet the 

 embryos originate from bodies which have a close similarity to 

 ordinary ova. 



Van Beneden has shewn that Dicyema consists in the adult 

 state of (i) a single layer of ciliated epiblast cells, somewhat 

 modified anteriorly to form a cephalic enlargement; and of (2) 

 one large nucleated hypoblast cell enclosed within the epiblast. 

 There are two kinds of embryo, both developed from germs 

 which originate in the hypoblast cell. The two kinds of embryo 

 arise in individuals of somewhat different forms. The one kind, 

 called by Van Beneden the vermiform embryo, arises in the 

 more elongated and thinner examples of Dicyema which have 

 been named Nematogens. These embryos pass directly into 

 the parent form without metamorphosis. 



The second kind of embryo, called infusoriform, is very 

 different from the parent, and has a free existence. Its eventual 

 history is not known. It originates in the shorter and thicker 

 individuals of Dicyema; which have been called Rhombogcns. 



The Vermiform Embryos. The germs or cells which give 

 rise to the vermiform embryos originate endogenously in the 

 protoplasmic reticulum of the axial hypoblast cell. They appear 

 as small but well-defined spheres, with a minute body in the 



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