606 



THE BIOLOGY OF MARINE ANIMALS 



chaete, the adults function solely as reproductive individuals, swim for a 

 while actively in the plankton, produce their eggs and die (65). 



Parasitism takes many bizarre forms but none is more unusual than that 

 displayed by the copepod Xenocoeloma brumpti (Herpyllobiidae), occurring 

 on the terebellid Poly cirrus arenivorus. Xenocoeloma appears like a cylindri- 

 cal sac attached to the side of the worm (Fig. 14.16). In reality, however, it 

 is an internal parasite, for it is completely covered by the host, except for 

 a terminal opening through which the ovaries discharge to the exterior. 

 Fusion of host and parasite tissues is most subtle : the epidermis of Xeno- 



Fig. 14.16. Parasitic Copepods 



(a) Pionodesmotes phormosomae ($) in an internal gall within the test of the urchin 

 Phormosoma uranus ; (b) Xenocoeloma brumpti attached to the body wall of Poly cirrus 

 arenivorus. ((a) after Koehler (1898); {b) after Caullery and Mesnil (16).) 



coeloma has disappeared and it is covered, instead, by epidermis of its 

 host, and the muscles of its body wall extend underneath the surface 

 epithelium of the worm. The head region and the alimentary canal have 

 disappeared, and the longitudinal axis of the parasite is occupied by a 

 diverticulum of the coelomic cavity of the host. Xenocoeloma is herma- 

 phroditic and the males have disappeared. The ovaries discharge into 

 oviducts, which become filled with developing eggs and which occupy a 

 large proportion of the mass of the animal. Terminally there are two large 

 testes which open into a seminal vesicle, from which the spermatozoa pass 

 into the oviducts and effect self-fertilization. The eggs mature on a pair of 

 ovigerous cords and hatch into nauplius larvae, devoid of alimentary 

 canal, and these infect new hosts (16). 



