608 THE BIOLOGY OF MARINE ANIMALS 



loss of locomotory appendages and sense organs, which cease to have 

 significance in fixed forms dependent on their hosts. The mouth parts 

 frequently become converted into piercing and sucking organs, as in 

 bopyrids ; and in internal parasites, which derive their food by absorption 

 of simple nutriments from the fluids of the host, there may be partial or 

 complete atrophy of the gut — for example, in cestodes and entoconchid 

 gastropods. The external surface of the animal may then assume the role 

 of the gut in absorption, and it is frequently augmented by means of folds, 

 ramifications and special appendages; for example, in rhizocephalans, 

 dendrogasterids and monstrillids. Corresponding to the reduction in 

 peripheral sensory and motor fields there is a diminution in the size and 

 complexity of central nervous organs. 



In those forms which are parasitic during only part of their life-history 

 it is found that regression is slight in the free-living stages, and becomes 

 most evident during the parasitic phase of the life-cycle, when a meta- 

 morphosis ensues. Nauplius larvae of peltogastrids and monstrillids, and 

 adult stages of the latter, are significant examples. The actively-swimming 

 stages, of course, are dependent upon an efficient sensori-neuromuscular 

 organization for their continued existence. Feeding, however, may be 

 suspended, the gut be atrophied and the animal be dependent on accumu- 

 lated food reserves obtained from the host. 



The female reproductive organs of parasites tend to become greatly 

 hypertrophied. Often the females are much larger than the males — for 

 example, in Bopyrus — in which a dwarf male is attached to the female. 

 The ovaries become very large and, in extreme cases, occupy the major 

 part of the body mass. A tendency towards hermaphroditism is also evident 

 in many groups. In Xenocoeloma the males have disappeared, and in 

 Sacculina the dwarf complementary males sometimes found on the body 

 of the female are vestigial individuals which no longer appear to be 

 functional. Self-fertilization is normal in many of these forms. Still other 

 species show successive hermaphroditism in that each individual develops, 

 first, the reproductive organs of one sex, and then gradually transforms 

 into the opposite sex later in its life-history; for example, cryptoniscids 

 and myzostomarians. 



The hypertrophy of reproductive structures in the female and the various 

 modes of hermaphroditism occurring among parasites are adaptations to 

 ensure perpetuation of the species under conditions in which mortality 

 of eggs and young stages is exceedingly high. Added to the normal hazards 

 of predation are the difficulties of encountering a suitable host animal 

 at the appropriate time and of securing access to the latter. The fecundity 

 of parasitic species is very great. Meyerhof and Rothschild (66), for ex- 

 ample, have recorded a case of a periwinkle Littorina littorea infected 

 with Cryptocotyle lingua, which over the course of twelve months emitted 

 about 1 ,300,000 cercariae. The complicated life-histories of heteroecious 

 parasites, which have one or two provisional hosts, can likewise be regarded 

 as adaptations for facilitating access to the final host (62). 



