VI CIRRIPEDIA 265 



longer, tapering, two-branched and many-jointed, appendages which serve 

 by their rhythmic movements to drive floating food-particles towards 

 the mouth. The mantle becomes strengthened by the formation of 

 strong white calcareous plates. In the ordinary Barnacles or Acorn- 

 shells (Balanus), the white shells which are seen in numbers on almost 

 every rock just below high-water mark, the form of body is conical but 

 in the pelagic stalked barnacles (Lepas), which live attached to floating 

 objects, the head region becomes much elongated to form a stalk on the 

 free end of which the body is borne. 



Various Cirripedes are parasitic in habit. Some of these — like the 

 large barnacles found embedded in the skin of whales — are comparatively 

 little modified, but others, such as Sacculina — a common parasite of 

 crabs — show the most extraordinary modification, the (hermaphrodite) 

 adult looking simply like a rounded bag projecting from the under side of 

 the abdomen of the host with root-like processes for the absorption of 

 nourishment branching amongst its internal organs. Here, however, as 

 in so many other similar puzzles in the animal kingdom the key to the 

 true relationships of the creature is at once given by the study of its 

 development, which shows (i) that the egg develops into a perfectly 

 typical nauplius larva, thus demonstrating it to be a member of the 

 Crustacea and probably of the Entomostraca and (2) that the nauplius 

 becomes transformed into a Cypris larva and so demonstrates that it 

 belongs to the Cirripedia. 



Phenomena of very great general interest and importance have been 

 observed in connexion with the influence of these parasitic cirripedes 

 upon their hosts, the metabolism of the host being upset in some peculiar 

 way that influences the sexual characteristics. The two sexes in Crabs 

 are normally recognizable at a glance, owing to the fact that in the 

 male the abdomen is narrow and devoid of appendages except a pair in 

 front modified for purposes of fertilization, while in the female the abdo- 

 men is broad and provided with four pairs of appendages for the attach- 

 ment of the eggs. Now in males infested with Sacculina it is found that 

 a large proportion take on the appearance of females and their testes 

 degenerate. After the Sacculina dies and disappears the gonad may 

 become again functional — with the remarkable peculiarity that it may 

 now produce eggs as well as spermatozoa : the whole constitution has 

 been caused to swing towards femaleness by the influence of the parasite. 

 Conversely in an infected female the constitution is caused to swing 

 away from femaleness as is shown by the abdominal appendages diminish- 

 ing in size but there is here no assumption of characters normally belonging 

 to the other sex. 



