354 
MON(ECIA — APHALLIA AND EXOPHALLIA. 
The Moncecia (fiovos, one ; olkos, house) in which group both sexes 
are combined in the same individual, is in its lowest forms, except in 
this respect, very little different from the Dioecia ; still in addition to 
the combination of both sets of organs, the group generally shows 
greater specialization and a remarkable development of accessory 
sexual organs. Hermaphroditism, according to Pelseneer, appears to 
be due to the grafting or superposition of the male system npon that 
of the female, and to have been apparently inflnenced in this direction 
by the adoption of terrestrial or fluviatile habits or parasitism. 
The increased bulk and complexity of the sexual organization, owing 
to the combination of the male and female organs within the body of 
each individual and the number of highly differentiated accessories 
that have been developed, is so marked that the greater part of the 
body cavity is occupied by them, especially during the pairing season, 
wlien they appear to dominate the whole organism. 
Yet, although both sexes are thus combined in each individual, the 
hermaphroditism is not complete, as conjugation with a second indi- 
vidual is normally always essential to ensure the fertilization of the 
ova and the development of the offspring. 
For the purposes of study, the Monoccia,like the Dioccia,may be sepa- 
rated into three chief groups : Aphallia, Exophallia, and Cryptophallia. 
The Aphallia (<i, not ; c/jaAAos, penis) are restricted to the Pelecy- 
poda in general, and include a few genera more especially fluviatile 
in habit, as Sp/ucriam, Pisidium, and certain species of Unionidw ; 
they, however, differ little from the Aphalliate Dimcia, e.xcepting that 
both tlie sexual elements are developed within the same individual, 
though the male and female gonads may occupy, as in the Spliceriidw , 
separate regions of the body, but as the ova and sperm are not 
usually developed simultaneously, the difficulty of appreciating their 
l)eculiarities is considerably increased especially when but one excre- 
tory duct is present. 
The Exophallia (f^o, external ; </)aAAo?, penis) are also closely 
similar to their Exo})halliate Dioecious kindred, except for the com- 
bination of male and female sexual organs within the body cavity of 
each individual, and tlie increase in number and complication of 
the su])plenientary organs. The Monocciate Exophallia are few 
in number, our only representatives being the species of the genus 
Valmta, in which group a primitive simplicity of general organization 
persists in spite of the acciuirement of the hermaphrodite sexual 
