FEEDING OF MARINE ANIMALS 211 



piercing and sucking organs by means of which they feed 

 on the blood and soft tissues of the fish. 



There are also marine leeches which suck the blood of 

 fish. The most exalted of marine parasites is the hagfish, 

 one of the Cyclostomes and a lowly ally of the true fish, 

 which fixes itself by a single tooth into the flesh of its 

 victim and then rasps away the tissues by means of its 

 scaly tongue, sucking in the food as it goes. 



The most degenerate types of parasites have completely 

 lost their feeding organs, they lie in the body cavity or 

 gut of their host bathed in nutrient fluid which they absorb 

 directly through the surface of their bodies. There are 

 innumerable examples of which only one — perhaps the most 

 striking — example can be mentioned here. It is quite 

 common to find on the under side of the bodies of various 

 kinds of crabs, a small round brownish mass — rather like 

 a tumour but in reality a parasite called Sacculina (Plate 

 80). It is practically without structure, just a bag contain- 

 ing reproductive products with a branching mass of roots 

 which penetrate the body of the crab in all directions and 

 absorb nourishment from it. Strangely enough this 

 parasite is a lowly relative of the animal in which it lives. 

 It is a crustacean most closely allied to the barnacles, a 

 fact which has been established by study of its life-history 

 (Plate 80). The eggs, which are discharged freely into the 

 sea from the parent parasite, hatch out into minute pear- 

 shaped creatures, each with three pairs of legs, a single 

 eye, and bearing a little shell exactly like the similar stage 

 of any other crustacean except that they have no stomach 

 or intestine. These minute " Nauplius " larva? (Plate 80) 

 swim about in the sea for some weeks when they moult 

 and become " Cypris " larva? which have a hinged shell 

 almost enclosing the body, a pair of feelers, and some six 



pairs of swimming legs. For a little time longer they 



? 



