IBLA AND SCALPELLUM. 235 



adapted to the different objects, to which the larva adheres. 

 Hence the antennae might, a priori, have been deemed of 

 high importance for classification. They are, moreover, em- 

 bryonic in their nature ; and embryonic parts, as is well 

 known, possess the highest classificatory value. From 

 these considerations, and looking to the actual facts as 

 exhibited in the following table, the improbability that 

 the parasites of 8. vulgar e and 8. Pero?iii, so utterly dif- 

 ferent in external structure and habits one from the other, 

 and from the Cirripedes to which they are attached, should 

 yet have absolutely similar prehensile antennae with these 

 Cirripedes, appears to me, on the supposition of the para- 

 sites being really independent creatures, and not, as I fully 

 believe, merely in a different state of sexual development, 

 insurmountably great. 



The parasites of 8. 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 in- 

 dependent parasite ; but in 8. ornatum the case is very dif- 

 ferent, 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 8. 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 through- 

 out the animal kingdom. 



