232 



ZOOLOGY 



are developed from the fertilised ova and subsequently escape, their develop- 

 ment being similar to that of the vermiform embryos : the phase of the parent 

 form to which they are developed is that known as the rhomboyene. After a 

 number of generations of males have been formed in this way, the rhombogene 

 undergoes modification, and the last generation of fertilised ova gives rise, not to 

 males, but to vermiform embryos — i.e., to an asexual generation — and with these 

 the cycle begins anew. 



The Heterocyemida', which are also parasites of the Cephalopoda, resemble 

 the DicyemidsB in most respects, but the head is wanting. 



The family Orthonectidce comprises only two genera — Rhopalura and 

 Stmcharthrum — which live as parasites in a Polyclad (Ltptoplana), a Neinertine 

 (Linens), an Annelid, and a Brittle- Star (Amphiura). In the stage that 

 represents the asexual form of the Dicyemidaj the Orthonectid assumes the 

 character of a plasmodium, or mass of finely granular protoplasm contain- 

 ing many nuclei, and is capable of active amoeboid movements. In the 

 interior of the plasniodia the sexual forms are developed. A nucleus of the 

 Plasmodium surrounds itself with protoplasm and gives rise to a germ-cell, 



Fio. 178.— Rhopalura Oiardii, male. 

 (From Brorm's Thierreicli, after Juliii.) 



Fin. 170.— Rhopalura Giardii, female. 

 (From Broiin's Thierreick, after Julin. 



which by a process of segmentation develops into the sexual stage. In some 

 cases only males are developed in one plasmodium and females in another : in 

 others both sexes are formed together in the same plasmodium. In some forms 

 the sexes are united. The sexual animals, especially the females (Fig. 179}, 



