PARASITES AND THEIR DEVELOPMENT. 



557 



long the barnacles, water-fleas, fish-lice, shrimps, 

 crabs, lobsters, etc. Most of these animals leave 

 the egg in the form of a " nauplius," and present 

 the closest possible resemblance to young saceu- 

 linae. The young of the fixed and rooted barna- 

 cles, which attach themselves like pseudo-parasites 

 to the sides of ships, so closely resemble young 

 sacculinaE that it would be a difficult, if not abso- 

 lutely impossible, task to separate or distinguish 

 the young from those of the sacculina in the ear- 

 lier stages of growth. The young barnacle, like 

 the young sacculina, resembles a shrimp of pe- 

 culiar kind on a roving commission, much more 

 closely than the adult and attached form. And 

 hence we discern in the common likeness of the 

 young of these animals a proof of their common 

 origin. At one time, therefore, we may believe 

 that the sacculina existed as a free-swimming 

 creature, of active habits, and possessing a toler- 

 ably high degree of organization. Doubtless some 

 less energetic member of the sacculina family se- 

 cured a temporary resting-place on the body of a 

 crab, and found such a position to be of desirable 

 kind from the rest and protection it afforded. 

 The feet, which were at first used for mere at- 

 tachment, may have come in time to penetrate 

 the body of the crab-host, and may thus have be- 

 come transformed into organs of nourishment. 

 By-and-by the sedentary life, with its advantages 

 in the way of cheap living and easy existence for 

 the sacculina, became a fixed habit. The saccu- 

 linae, which acquired this habit, together with their 

 descendants, would flourish and increase in num- 

 bers, owing to the advantage gained by them in 

 that " struggle for existence" in which sacculina} 

 and their highest animal neighbors are forced, 

 one and all, to take part. And as the wholly free 

 sacculinas became transformed into higher forms 

 of life, or became extinct, their rooted and para- 

 sitic brethren may be regarded as having gradual- 

 ly degenerated. A process of physiological back- 

 sliding invariably takes place in such cases, and 

 this retrogression would be manifested in the sac- 

 culinse by the casting off of structures which were 

 no longer of use to a fixed and rooted being — the 

 degeneration and disappearance of structures not 

 required in the animal economy, taking place in vir- 

 tue of the well-known law of the "use and disuse" 

 of organs. The legs would thus become gradually 

 diminished, and would finally disappear altogeth- 

 er. Internal organs, and parts useful to the free- 

 swimming animal, would become useless as the 

 creature became more and more dependent on its 

 host. And finally the sac-like organism would be 

 evolved as the result of its parasitic habits ; and 



the degeneracy which marks the slavishly depend- 

 ent mind in higher life is thus viewed as also de- 

 stroying the independence and as warping and 

 distorting the character which once marked the 

 free and active creature of lower grade. Thus 

 we may understand by the study of life-histories, 

 such as those of sacculina and its comrades, how 

 parasitism is induced, and how a change of life 

 and habits of such sweeping character, convert- 

 ing an active being into a sedentary and degraded 

 animal, becomes established through the slow but 

 sure effects of habit, use, and wont, perpetuated 

 through many generations. 



Perhaps the most inveterate and dreaded ene- 

 my which man has to encounter in the ranks of 

 parasites is the little Trichina, which has on more 

 than one occasion caused a fatal epidemic, on the 

 Continent especially, through its development in 

 excessive numbers. This little worm-like parasite 

 was first discovered in the dissecting-room of 

 St. Bartholomew's Hospital. The circumstances 

 of its discovery have been frequently repeated in 

 anatomical rooms by the observation that very 

 small hardened bodies are to be sometimes met 

 with imbedded among the muscular tissue of the 

 human subject. When one of these little bodies 

 is carefully examined, it is found to consist of a 

 little sac or bag of oval shape, and to contain 

 within a little worm coiled up in a spiral fashion. 

 These sacs attain a length of about one-seven- 

 tieth of an inch or so, and, if they have existed 

 within the muscles for a lengthened period, they 

 will be found to be somewhat limy in structure ; the, 

 presence of this mineral implying degeneration 

 of the sac and its tenant. When the first trichina 

 were examined and named by Prof. Owen, their 

 life-history and importance, as regards the human 

 economy, were unknown and undreamed of. But 

 the occurrence on the Continent of certain mys- 

 terious cases of illness, and the careful investiga- 

 tion of such cases by medical men, led to the 

 recognition of the fact that this tiny worm, which, 

 in its fully-grown condition, does not exceed a 

 mere fraction of an inch, may nevertheless, 

 through its development in large numbers, prove 

 a source of fatal disease to man. In proof of 

 this fact we may quote Dr. Cobbold's extract 

 from the Leipziger Zeitung for December 8, 1863, 

 in which it is stated that six persons were seized 

 with all the symptoms of trichina-disease, " after 

 eating raw beef mixed with chopped pork." The 

 Neue Hannoversche Zeitung for December 13th 

 of the same year chronicles the death of twenty- 

 one persons in Hettstadt through eating the flesh 

 of an English pig, the butcher himself perishing 



