tulum, thorax, and cirri, are in a quite extraordinary state of
atrophy; in fact, the parasitic males of Ibla consist almost exclusively
of a mouth, mounted on the summit of the three anterior segments of the
21 normal segments of the archetype crustacean. In the males of the
first three species of Scalpellum, some of the characters are
embryonic,--as the absence of a mouth, the presence of the abdominal
lobe, and the position of the few existing internal organs; other
characters, such as the general external form, the four bead-like
valves, the narrow orifice, the peculiar thorax and limbs, are special
developments. These three latter parasites, certainly, are wonderfully
unlike the hermaphrodites or females to which they belong; if classed as
independent animals, they would assuredly be placed not in another
family, but in another Order. When mature they may be said essentially
to be mere bags of spermatozoa.

In looking for analogies to the facts here described, I have already
referred to the minute male Lerneidae which cling to their females,--to
the worm-like males of certain Cephalopoda, parasitic on the
females,--and to certain Entozoons, in which the sexes cohere, or even
are organically blended by one extremity of their bodies. The females in
certain insects depart in structure, nearly or quite as widely from the
Order to which they belong, as do these male parasitic Cirripedes; some
of these females, like the males of the first three species of
Scalpellum, do not feed, and some, I believe, have their mouths in a
rudimentary condition; but in this latter respect, we have, amongst the
Rotifera, a closely analogous case in the male of the Asplanchna of
Gosse, which was discovered by Mr. Brightwell[62] to be entirely
destitute of mouth and stomach, exactly as I find to be the case with
the parasitic male of _S. vulgare_, and doubtless with its two close
allies. For any analogy to the existence of males, complemental to
hermaphrodites, we must look to the vegetable kingdom.

Finally, the simple fact of the diversity in the sexual relations,
displayed within the limits of the general Ibla and Scalpellum, appears
to me eminently curious; we have (1st) a female, with a male (or rarely
two) permanently attached to her, protected by her, and nourished by any
minute animals which may enter her sack; (2d) a female, with successive
pairs of short-lived males, destitute of mouth and stomach, inhabiting
two pouches formed o