152 LIFE : OUTLINES OF GENERAL BIOLOGY 



In Sacculina the absorptive outgrowths ramify like roots right 

 through the body of the parasitised crab. The head of the pigmy 

 parasitic male of the mid-water Angler has grown continuous with 

 the tissue of the female who carries him. A strange Gasteropod 

 parasite (Kntoconcha) has its head thrust into a blood-vessel of 

 its holothurian host. When the available food from the host is very 

 abundant, it maybe utilised by the parasite in a somewhat uneco- 

 nomical fashion; thus some Nematodes ferment glycogen into valeri- 

 anic acid, carbon dioxide, and hydrogen, which is far from making 

 the most of the material. In tapeworms the whole surface of the 

 body absorbs liquid food. Parasites deeply imbedded in tissues must 

 be nourished by the lymph, just as if they were parts of the host. 



(3) It is characteristic of many thoroughgoing endoparasites that 

 they can survive in conditions where free oxygen is apparently 

 ver>' scarce. In most cases they seem to obtain a sufficient supply 

 from the blood or tissues of their host, just as if they were parts of 

 the body. There is no modern corroboration of the older view that 

 some nuilticellular parasites can live anaerobically. 



(4) Many parasites, such as Nematodes, Crustaceans, Insects, 

 and Mites, have a chitinous cuticle, which is very resistant, e.g. to 

 bacteria and to digestive juices. But the presence of a thin-skinned 

 tapeworm in man's intestine or of delicate Infusorians in the horse's 

 stomach requires some explanation, and there is some evidence that 

 their insusceptibility to the digestive juices is due to their develop- 

 ment of an "anti-body". 



(5) There are some noteworthy adaptations in connection with 

 reproduction. Thus resistant egg-shells are characteristic of Platy- 

 helminths, and the tapeworm's liberation of an entire joint, capable 

 of some independent movement, must often be advantageous. 

 The difficulty of securing fertilisation, when the parasites do not 

 occur in large numbers together, is met in various ways, e.g. {a) by 

 a very prolonged association of the sexes, as in Bilharzia, where the 

 male carries the female, or in Chondracanthus, where the female 

 carries the male; {b) by self-fertilisation or autogamy, as in the 

 liver-fluke; or (c) by an emergence of the sexually mature forms 

 into freedom, as in Horsehair-worms (Gordiacea). There are some 

 very remarkable ca.ses, notably the Trematode, Diplozoon para- 

 doxtim, where two mature hermaphrodite individuals are united in 

 I emianent cross-fertili.sation. In the genus Wedlia, two individuals 

 are found together inside a cyst, a smaller one — the male — imbedded 

 into a protrusion of the vesicular posterior body of a larger one, the 

 female. But each individual shows traces of the gonads of the 

 opposite sex, so that this looks like a secondary abandonment of 

 hermaphroditism, when arrangements for securing fertilisation had 

 b<('n in the course of time established. Quite another circumvention 

 of the difficulties is a relapse into parthenogenesis, as in some 



