NO. 10 CRUSTACEAN METAMORPHOSES — SNODGRASS 25 



As an example of the life history of a parasitic copepod that returns 

 to a free life in the adult stage, we may take the monstrillid Cyniba- 

 softia rigidum Thompson, described by Malaquin (1901) as Haemo- 

 cera danae (Claparede), which in its larval stages lives in the blood 

 vessel of the serpulid worm Salmacina dysteri Huxley. The nauplius 

 (fig. 9 A) has the usual three pairs of naupliar appendages, but the 

 mandibles are recurved hooks, and the young larva has no mouth or 

 alimentary canal. It is poorly fitted for swimming, and Malaquin 

 suggests that the females probably sow their eggs over a colony of 

 the serpulids. When in contact with a worm the nauplius attaches 

 itself by its mandibular hooks to the worm's integument, but it has 

 no special organs for penetration. The skin of the worm, however, 

 is delicate, and, a puncture once effected, the nauplius does a most 

 surprising thing; it casts off its own cuticle and its appendages and 

 forces its soft nude body into the host. Within the latter it becomes 

 a shrunken, oval mass of undifferentiated cells (B), as if it had re- 

 turned to an early embryonic condition to begin development all over 

 again. In this form the parasite traverses the coelom of the host and 

 makes its way into the ventral blood vessel. Here it secretes a new 

 cuticle and then from its ventral side anteriorly there grow out two 

 tapering, armlike processes (C) that extend posteriorly in the blood 

 vessel of the worm and will serve the parasite as food-absorbing 

 organs. Here, therefore, we see a metamorphic development adapting 

 the parasite to its life in the host that certainly had no counterpart in 

 the presumed free-living ancestors of its species. It is hard enough 

 to believe the facts themselves, and we can speculate in vain as to 

 how they all came about in evolution. The nauplius is prepared in 

 advance for the life it is to lead by being provided with hooked 

 mandibles, but what induces it to shed its cuticle and appendages and 

 to squeeze itself into the worm? 



With the growth of the young larva in the worm (fig. 9H) the 

 nutritive arms increase in length (D, E), the new cuticle is drawn 

 out into a rostrum in front (E, R), and on the enlarged conical 

 posterior part of the body it becomes armed with circles of spines 

 directed forward. Tlie organs of the future adult now gradually de- 

 velop within the cuticle of the larva (F), and the abdomen forms as 

 a ventral flexure {Ah) of the posterior part of the body. At an early 

 stage the first antennae are regenerated (F, G, lAnt) and eventually 

 penetrate into the rostrum (I) when the head tissue has receded from 

 the latter. From this point on the parasite develops normally into the 

 adult form within the cuticular sheath of the larva. Finally, when its 

 development is almost completed (I), the parasite becomes strongly 



