The parasites of _S. vulgare_ take advantage of a pre-existing fold on the edge of the scutum, where the chitine border is thicker; and in this respect there is nothing different from what would naturally happen with an independent parasite; but in _S. ornatum_ the case is very different, for here the two scuta are specially modified, _before the attachment of the parasites_, in a manner which it is impossible to believe can be of any service to the species itself, irrespectively of the lodgment thus afforded for the males. So again in _S. rutilum_, the shape of the scutum seems adapted for the reception of the male, in a manner which must be attributed to its own growth, and not to the pressure or attachment of a foreign body. Now there is a strong and manifest improbability in an animal being specially modified to favour the parasitism of another, though there are innumerable instances in which parasites take advantage of pre-existing structures in the animals to which they are attached. On the other hand, there is no greater improbability in the female being modified for the attachment of the male, in a class in which all the individuals are attached to some object, than in the mutual organs of copulation being adapted to each other throughout the animal kingdom. Generic Characters of the larval prehensile ANTENNÆ, in the Lepadidæ, as far as known from their imperfect state of preservation, and the number of species examined. |Name of Species. | |Length of, from end of disc to the further margin of the | |oblique basal articulation: Scale, fractions of the 1/6000ths | |of an inch. | | |Length of, from end of disc to the inner margin of the | | |basal articulation. Scale same. | | | |Width of basal segment, in widest part. Scale same. | | | | |Disc, length of. Scale same. | | | | | |Disc, width of. Scale same. | | | | | | |Ultimate segment, length of. Scale | | | | | | |same. | | | | | | | |Ultimate segment, width of. | | | | | | | |Scale, fractions of the | | | | | | | |1/20,000ths of an inch. LEPAS: disc large, thin, almost _circular_, slightly elongated, with several long spines on the hinder margin; end segment with three very long, plumose spines on the upper _exterior_ angle.[61] |_L. anatifera_ (?) | |62 | | |-- | | | |20 | | | | |23 | | | | | |22 | | | | | | |-- | | | | | | | |-- |_L. australis_, | |111 | | |-- | | | |40 | | | | |42 | | | | | |39 | | | | | | |18 | | | | | | | |30