CHAPTER IV. 



DICYEMID^E AND ORTHONECTIDyE. 



DlCYEMID^E. 



THE structure and development of these remarkable parasites 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 (1) 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 ex- 

 amples 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 Rhombogens. 



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 centre. In these 

 spheres a cortical layer becomes differentiated, which gradually 

 increases in thickness and gives rise to the body of a cell, the nucleus 

 and nucleolus of which are respectively formed from the inner part 

 of the original sphere and the minute central body. These germs 

 can originate in all parts of the hypoblast cell and are frequently very 

 numerous. 



The germ when completely formed undergoes a segmentation 



