Dr. Baird on Entozoa, or Intestinal Worms. 145 



to prove his theory of the alternation of generations, he shows 

 that several of the species of Distoma, or fluke, which we find in 

 the liver of some of our freshwater mollusks, are, previous to theii 

 taking possession of their habitat, in the body of the snail, free 

 and unattached, enjoying the power of locomotion in the water 

 in which the mollusks themselves are found. In this stage of 

 growth they have been described as a distinct genus, under the 

 name of Cercaria. These Cercaria he has often seen attach 

 themselves to the body of the snail, get rid of the tail which in 

 their free state they possessed, and after a short time make for 

 themselves a circular cavity in the mucous integument of their 

 host. The mucus gradually hardens, and forms a tough, nearly 

 transparent case around them, in which they lie concealed for 

 months in the state of a pupa. Emerging from this pupa state, 

 they penetrate deeper into the skin, and following the course of 

 the aqueous canals of the snail, they at length enter the organs 

 in which as true Distomata they are found. In the same manner 

 this ingenious naturalist accounts for several of the Entozoa oc- 

 curring in the eyes of fishes. At first swimming free and unat- 

 tached in the water, they after a time fasten themselves to the 

 cornea, or in the immediate neighbourhood of the eye, of the 

 fishes they are destined to inhabit. Afterwards assuming the 

 pupa state, they make for themselves a nidus under the skin, or 

 under the cornea, where they remain for a certain period of time, 

 till, emerging from their pupa state, they are able to penetrate 

 into the substance of the eye itself. We thus see that the origin 

 of the Entozoa is a subject abounding in extraordinary interest ; 

 and I hope some of the Members of the Club, who may have 

 opportunities of observation, may be induced to devote some of 

 their time and attention to it. 



II. The Evils they inflict upon Mammalia and other Animals. 



I will not enter at present upon the subject of the mischiefs 

 which man suffers from the presence of the Taenia, or tape-worm, 

 the Ascarides, &c, in the intestinal canal ; the encysted Entozoa, 

 such as the Echinococcus, in the liver, &c, these being subjects 

 for the investigation more particularly of the physician ; but shall 

 confine my remarks to a few instances of the extensive ravages 

 they sometimes commit upon the lower animals. 



The celebrated Camper gives a remarkable instance of the 

 fatal effects produced in young cattle by the introduction of a 

 species of Strongylus into the trachea and lungs. An epizootic, 

 which destroyed many thousands of cattle, appeared in Holland 

 in 1768, and threatened the destruction of the whole stock of 

 horned cattle in the country. Camper's services were called into 

 requisition, by his countrymen, and inoculation, which had been 



