,CHAP. XII. EVOLUTION OF THE EHIZOCEPHALA. 137 



indicate homologous parts of the body. From the 

 point of attachment in the Ehizocephala the roots pene- 

 trate 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 ways in different 

 species. The cement-ducts in the basis of the Balanidae 

 likewise constitute a generally remarkably complicated 

 system of ramified tubes, with regard to the mode of 

 termination of which nothing certain has yet been 

 made out. Individual csecal branches are not unfre- 

 quently 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 con- 

 verted by natural selection into Khizocephala ? 



A considerable number of existing Cirripedia settle 

 exclusively or chiefly upon living animals ; on Sponges, 

 Corals, Mollusks, Cetaceans, Turtles, Sea-Snakes, Sharks, 

 Crustaceans," Sea Urchins, and even on Acalephs. Di- 

 chelaspis Darwinii was found by Filippi in the bran- 

 chial cavity of Palinurus vulgaris, 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, 



