168 THE STORY OF LIFE IN THE SEAS. 



There is a remarkable parasite called Sacculina 

 which may sometimes be found on the under side 

 of the tail in Crabs. In shape it is like a small 

 pea or bean, and is attached to its host by a 

 number of root-like processes, which penetrate 

 through the skin and burrow deeply into the 

 subjacent tissues. It would be quite impossible 

 to tell to what group of animals this parasite 

 belongs by the study of the adult form alone. 

 It is, in fact, little more than a skin full of eggs. 

 When the development of the eggs is watched, 

 however, it is observed that the young Sacculina 

 as it is hatched is very much like the Nau- 

 plius larva of a Barnacle. The later stages of the 

 development prove that whatever may happen 

 to the adult the Sacculina must be related to the 

 group of the Cirripedia. Later on it is found 

 that the females settle down on a Crab, lose all 

 their limbs and other Cirripedian characters, and 

 finally degenerate into a mere palpitating sac of 



The males never pass beyond the second stage 

 of development known as the Cypris stage. 

 Several of them may usually be found attached 

 to the female, and although they always remain 

 extremely minute they do not lose entirely their 

 Crustacean features. 



For those who are in search of parasites, how- 

 ever, there is no more fruitful ground than the 

 gills. That these organs should be a good place 

 for attack is not surprising, when we consider 

 that to maintain the respiration of the animal a 

 constant flow of sea-water over them must be 

 kept up, and this must bring with it many larval 



