Parasitism and Social Reciprocity. 

 vibratile cilia by which they propel themselves through the water after 

 the manner of an animal. 



The case of the molluscous parasite Entoconcha Mirabilis, is even 

 more remarkable than the crustacean parasites named. The larva 

 of this mollusk has an oval shell with an operculum, or cover, to fit the 

 mouth ; it has a "velum," or sail, or organ for swimming such as pos- 

 sessed by many similar larva of the univalve mollusks ; it has a gill cav- 

 ity and intestines, an auditory organ and a correlating nerve ganglion, 

 and, in short, is equipped exactly to all appearance like the larvae of 

 those univalve mollusks that remain free all their lives. But this one 

 becomes a parasite in a Holothurian (sea-slug), and as he matures here 

 he loses everything his shell and cover, his swimming organ, his ear 

 and his piece of brain, and, in short, he is converted into a mere para- 

 sitic pouch, saving nothing but the hermaphrodite male and female 

 glands and rearing the embryos of the next generation. 



The same food is not appropriate for all the stages of the life of an ani- 

 mal, as a general rule. In the case of mammals, the } T oung for a year or 

 two live on milk and later become adapted to their adult food of grass, 

 grain, fruit, meat, &c. In the case of the young parasites, in order to 

 obtain the progressive food supply, the animal must migrate, since its 

 parents from their extremely shiftless habits are very poor providers. 

 It is this necessity of going to the food, since the food cannot be 

 brought to them, that develops the activity and enterprise of the 3 r oung 

 of parasites and sedentaries, as such animals as the Barnacle may be 

 called, while the infants of active parents are relatively helpless, as in 

 the case of mammals, especially the primates. This necessity is further 

 illustrated in the case of the entoparasites or those living within the 

 body of the host. The Trichina Spiralis, a nematode worm, gains en- 

 trance into the body of an animal a pig, for instance by being eaten. 

 In the stomach of the pig the Trichinae are set free from the muscle in 

 which they had been confined as the muscle is being disintegrated in the 

 process of digestion. In the space of three days the Trichinae grow to 

 maturity, and become sexual, the females being more numerous than the 

 males. They possess a mouth and intestine and sexual glands. The 

 females then lay a prodigious number of eggs, from each of which is 

 hatched a microscopic }*oung Trichina. These bore through the walls of 

 the stomach or intestines and find their way to the muscles of their host, 

 where they take up their lodgings, each one coiled up in a minute cavity 

 or cyst formed between the fibres of the muscle. There their journey 

 and their activities cease. They cannot lay their eggs in the muscle, 

 neither can they go back to the stomach they came from to finish their 

 growth and lay their eggs there. They may remain alive ten years 

 where they are, but will be entirely inactive until the 'muscle containing 



