144 in-E : OUTLINES OF GENERAL BIOLOGY 



of tlic Eastern States— a parallelism so close as surely to l)e of evo- 

 lutionary suggest iveness, for these two parasites seem to be genuine 

 members of the earthworm order (Oligochsta), though unlike these 

 they are without locomotor bristles, and hold on by suckers and bite 

 with chitinous jawplates. In these two res|x?cts, they exhibit a 

 convergence towards leeches, no doubt in adaptation to a similar 

 mode of life. Leeches (Ilirudinea) themselves should be regarded as 

 predatory, but incipientiy parasitic. Land-leeches often exert 

 themselves to effect attachment to passing animals, as even man 

 sometimes finds to his cost. Certain perplexing forms (Mysostoma) 

 — |)erhaps offshoots from primitive Annelids— which form galls on 

 Crinoids, are much degraded in relation to their parasitic mode of 

 life. A few Rotifers (e.g. Albertia) are parasitic in or on freshwater 

 worms. Another (Seison) occurs on an interesting primitive marine 

 crustacean (Nebalia); Discopus attaches itself to a holothurian 

 (Synapta) ; and Callidina parasitica to the limbs of some freshwater 

 crustaceans (Gammarus and Asellus). But the reality of the para- 

 sitism requires reinvestigation in most cases; the linkage seems 

 sometimes nearer that of commensalism. 



While there are no parasitic Echinoderms, the habit is common 

 among the lower Crustaceans, especially among Co{>epods, many 

 of which are called "fish-lice" (e.g. Caligus and Lerntea). Many 

 grades occur, for tlie association may be temporary or permanent 

 (after the early free larval stages), and it is often confined to the 

 females. On the other hand, the male may be reduced in size, and 

 even become a parasite on the female, as in Chondracanthus. Allied 

 to the barnacles (Cirripedia) are the peculiar Rhizocephala, e.g. 

 Sacculina already mentioned, which is so often to be noticed by 

 shore-collectors as a bean-like protrusion from the ventral surface 

 of the abdomen of common shore crabs, as well as on our edible 

 species. Its story has been so well made out by that admirable marine 

 zoologist Delage, that it is well worth following in some detail. 

 It starts life as a free-swimming nauplius larva, it develops into a 

 Cyprid stage, and fixes itself to the back of a young crab, and pierces 

 the small circle of uncalcified membrane around the base of one of 

 the few large bristles of this region. It then loses almost all but its 

 head, the thorax and abdominal regions degenerating to form a 

 minute tube through which the head then slides (somewhat as om 

 may do with the tip of an inturned glove finger!) The head thu 

 sinks into the crab, and eventually, as maturity is approached, what 

 remains of the body tube grows outwards, and protrudes on the 

 abdomen. The full-grown sac consists mainly of hermaphrodit< 

 reproductive organs, nourished by numerous absorbent root-lik« 

 processes which spread from the otherwise degenerate head, and 

 l)enetrate deep into and through the tissues of the crab. Sacculina 

 lives for about three years, arresting the crab's growth, and in the 



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