6 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 123 



Altho 38 of the 43 species dealt with by Hansen in his classic 1897 

 monograph inhabit the marsupia of Amphipoda, Isopoda, Cumacea, 

 and Mysidacea, Hansen did not call attention to the similarity in size 

 between the eggs of the host and the ovisacs of the choniostomatid 

 parasites. The above peracarid crustaceans aerate and keep their 

 eggs free of contaminating particles by circulating a current of water 

 thru the marsupium by movements of the pleopods (gammaridean 

 Amphipoda), maxillipeds (Isopoda, Cumacea), or oostegites (Tanai- 

 dacea, Isopoda, caprellid Amphipoda) . There is no flexible appendage 

 that can reach into the marsupium and handle the eggs individually, 

 except possibly in the Mysidacea (Jancke, 1924); hence, mimicry of 

 the eggs of these Crustacea is not needed to prevent them from ejecting 

 the eggs of their choniostomatid parasites. Individual copepod eggs, 

 however, because of their small size, would be in danger of being 

 flushed from the marsupium by the circulating water current, whereas 

 ovisacs similar in size to the host's eggs are too large to encounter this 

 risk. Thus it is advantageous to the choniostomatid to lay its eggs in 

 groups rather than singly. 



A further advantage to the parasite in having its ovisacs resemble 

 the host's eggs is that the parasitized host does not become more 

 conspicuous and thus more vulnerable to predators than in the normal 

 ovigerous condition. This advantage only operates when the mar- 

 supial contents are visible, and is probably much less significant in 

 ostracods, in which the shell obscures the eggs, than in crustaceans 

 with transparent oostegites. Delia Valle (1893) pointed out that the 

 ovisacs of species of Sphaeronella infesting the amphipods Ampelisca 

 diadema and Microdeutopus gryllotalpa matched the color of the host's 

 eggs. This predator-deceiving egg mimicry, to be distinguished from 

 the host-deceiving egg mimicry in Sphaeronellopsis monothrix, is also 

 characteristic of marsupial parasites that are too large or firmly 

 attached to be dislodged by the host. Crytoniscid isopods (species 

 summarized recently by Nielson and Stromberg, 1965) are in this 

 category. Whether the host behaves normally in ventilating and 

 cleaning the contents of its marsupium when it is occupied by a 

 predator-deceiving parasite instead of its own eggs is not known. 



Sphaeronellopsis Hansen, 1905 



Sphaeronellopsis monothrix Bowman and Kornicker, new species 



Figures 1-3 



Female. — Length .23-.47 mm. Head well defined from trunk, 

 relatively large. Trunk wider than long in fully developed female, 

 narrower than long in young female, ventral surface with a number 

 of rows of minute hairs. Submedian skeleton of head well developed; 



