262 



Fur Darwin. 



its end was torn away, and in Dichelaspis, judging from its ragged appearance, 

 it had attached itself firmly to the support. From this it follows that this append- 

 age in Lepas australis can hardly be anything but a young cement-duct. If, 

 therefore, the supposition that the appendages on the antennae of the pupae of 

 Rhizocephala are young roots be correct, the roots of the Rhizocephala are homo- 

 logous with the cement-ducts of the Cirripedia. And this, strange as it may 

 appear at the first glance, seems to me scarcely doubtful. It is true that the act 

 of adhesion of the Rhizocephala has never yet been observed, but it is more 

 than probable that they attach themselves, just like the Cirripedia, by means of 

 the antennae, and that therefore the points of attachment in the two groups indicate 

 homologous parts of the body. From the point of attachment in the Rhizocephala 

 the roots penetrate into the body of the host, whilst in the Cirripedia, the cement- 

 ducts issue from the same point. The roots are blind tubes, ramified in different 

 species. The cement-ducts in the basis of the Balanidae likewise contitute a gene- 

 rally remarkably complicated system of ramified tubes, with regard to the mode 

 of termination of which nothing certain has yet been made out. Individual caecal 

 branches are not unfrequently seen even in the vicinity of the carina; and, at 

 least in some species, in which the cement-ducts divide into extremely numerous 

 and fine branchlets, forming a network which gradually becomes denser towards 

 the circumference of the basis, these seem nowhere to possess an orifice. 



Now as to the question: How were Cirripedia converted by natural selection 

 into Rhizocephala? 



A considerable number of existing Cirripedia settle exclusively or chiefty 

 upon living animals; - - on Sponges, Corals, Mollusks, Cetaceans, Turtles, Sea- 

 Snakes, Sharks, Crustaceans, Sea Urchins, and even on Acalephs. Dichelaspis 

 Darwinii was found by Filippi in the branchial cavity of Palinnrus vulgar is, 

 and I have met with another species of the same genus in the branchial cavity 

 of Lupea diacantha. 



The same thing may have taken place in primitive times. The supposition 

 that certain Cirripedes might once upon a time have selected the soft ventral 

 surface of a Crab, Porcellana or Pagurus, for its dwelling-place, has certainly 

 nothing improbable about it. If then the cement-ducts of such a Cirripede instead 

 of merely spreading on the surface, pierced or pushed before them the soft ventral 

 skin and penetrated into the interior of the host, this must have been beneficial 

 to the animal, because it would be thereby more securely attached and protected 

 from being thrown off during the moulting of its host. Variations in this direction 

 were preserved as advantageous. 



But as soon as the cement-ducts penetrated into the body-cavity of the host 

 and were bathed by its fluids, an endosmotic interchange must necessarily have 

 been set up between the materials dissolved in these fluids and in the contents 

 of the cement-ducts, and this interchange could not be without influence upon 

 the nourishment of the parasite. The new source of nourishment opened up in 

 this manner was, as constantly flowing, more certain than that offered by the 

 nourishment accidentally whirled into the mouth of the sedentary animal. The 

 individuals favoured in the development of the cement-ducts now converted into 

 nutriferous roots, had more than others the prospect of abundant food, of vigorous 



