ECOLOGICAL 137 



crab, but its root-like absorptive outgrowths penetrate through and 

 through its host, and its bean-Hke adult stage is actually a bulging 

 out like a hernia. The larval stage of SaccuHna is a free-swimming 

 nauplius. It is thus not such an easy matter as it seems to distinguish 

 between outside and inside; for when an ectoparasite is sedentarily 

 fixed to the skin and yet absorbs food by an intruded portion of its 

 body, it partakes of the nature of both, and combines their advan- 

 tages. Yet we hardly think of a sedentary plant-mite as an endo- 

 marasite, though it too may permanently insert its head into the 

 Ivictim. Moreover, it becomes, as will be seen, no easy matter to 

 [decide whether small animals, such as other mites, that wander 

 ibout on the surface of an animal's body, are to be regarded as 

 )arasites or not. Some are only scavengers; others draw blood; 

 id others deposit eggs in the skin of their host. On the whole, 

 lowever, there is a general distinction between ectoparasites and 

 idoparasites. 



fEPIPHYTIC AND EPIZOIC RELATIONS.— Parasitism must be 

 istinguished from epiphytic or epizoic relations. An epiphytic 

 )lant grows on another plant without deriving any nourishment 

 rom it, as in the case of orchids perched on trees, or of the green 

 ilgae and lichens on bark. Similarly microscopic green Algae live on 

 |the surface of the coarse hairs of the BraziHan tree-sloths, and many 

 seashore crab carries a garden of seaweeds or an incrustation of 

 barnacles upon its shell. But if the crab, such as Hyas araneus, 

 has itself implanted these Algae, and if there is evidence that the 

 Crustacean is usefully masked, while the plant is benefited by being 

 carried about, then the relation passes into commensalism, i.e. a 

 mutually beneficial external partnership between two organisms of 

 different kinds. 



An epizoic animal may live attached to another animal without 

 deriving any nourishment from it, as a bunch of barnacles may be 

 attached to the flattened tail of a sea-snake, or as a Tunicate, a 

 False Oyster (Anomia), a Serpulid worm, a Polyzoon colony, and a 

 Sponge may all be found together on the shell of a Whelk. But if 

 the sponge (e.g. Suberites) should mask a hermit-crab ensconced in 

 the empty shell of a periwinkle, 'and should be benefited by its 

 association with the vigorously active animal, then the epizoic 

 relation becomes a commensalism. Various marine animals, such as 

 hydroids and even sea-anemones, hve attached to large Laminarian 

 seaweeds, but without any nutritive relation, cases which have to 

 be distinguished from animals that habitually browse on the sea- 

 weed, like the beautiful "pellucid limpet" [Helcion pellucidum). 

 Freshwater sponges are often epiphytic on the stems of aquatic 

 plants in rivers and lakes; and by the sides of the Amazon, when the 

 water is low, they may be seen at a considerable height on trees ! 



