142 THE BIOLOGY OF THE SEA-SHORE 



All parts of the host are liable to infection, but parasites 

 are most general in the alimentary canal ; they are frequent 

 also in the swim-bladders of fishes, the lungs, liver, body 

 cavity, and serous spaces, bladder, sex organs, heart, blood- 

 vessels, eye or brain of various animals, or encysted in the 

 skin, connective tissues, muscles, or nervous system. (See 

 Ward, 191 8.) In fact, no organ of their Invertebrate or 

 Vertebrate hosts is immune. Parasites are, however, most 

 common in Vertebrates and Insects, Crustacea and worms 

 being less frequently infected ; while the chief groups 

 supplying parasitic species are the Protozoa, Platyhelminthes 

 (Flukes and Tapeworms), Nematoda (Round- worms and 

 Thread- worms), and Arthropoda (Insects, Mites, Ticks, etc.) ; 

 but nearly all the other invertebrate groups show parasitic 

 forms also. With regard to the origin of the complex forms 

 of parasitism with alternation of hosts, it has been suggested 

 among other theories that the present intermediate host is 

 possibly the original one, and at one time harboured both 

 larval and adult stages. In time, as evolution proceeded and 

 other and higher animal forms were produced, later stages 

 of the parasites would have sought out other hosts from 

 among these higher animals. If this is correct we are able 

 to understand how it is that a particular parasite has its 

 larval stage in an Invertebrate animal, and its adult stage 

 in a Vertebrate form, as so often happens (Fantham, 

 Stephens, and Theobald, 191 6). 



The commonest shore parasites belong to one or another 

 of two groups, viz. the " worms " and the Crustacea. Among 

 the latter, the form which most frequently comes under the 

 notice of the observer is Sacculina, This well-known para- 

 site of the common shore crab {Carcinus moenas) degenerates 

 into a sac-like mass of gonads which is external and a root- 

 system which penetrates every part of the body of the crab 

 and draws nourishment therefrom. The adult parasite bears 

 no resemblance whatever to a Crustacean, and it is only the 

 fact of its passing through a naupUus and a cypris stage in the 

 course of its development that enables us to class it, along 

 with the common rock-barnacle, as a Cirripede. The 



