INSECTS INFESTING DOMESTIC ANIMALS. 359 



Redid, ov second lava form, has acquired its complete growth, 

 it is somewhat of an arrow-head shape, consisting of a sac, 

 within wliich is suspended a tubular bag containing colored 

 masses, which Huxley supposes are alimentary. The head is 

 represented by a kind of a crown, and near the other extrem- 

 ity are two hxteral projections. In the l)ody cavity, external 

 to the tabular sac, vesicles now appear, which rapidly increase 

 and assume the form of Cercarise, the name given to the third 

 larval stage. The Redia now bursts and these new zooids 

 escape. The multij)lication at an intermediate and incomplete 

 stage (before sexual characters have appeared) is very remark- 

 aljle and introduces to our view a strange feature in animal 

 life. 



" The Cerraria resembles a peanut, with a slender tail attached 

 to one end ; it also has lateral membraneous attachments, by 

 means of which it swims after the manner of a tadpole. After 

 swimming free for a certain length of time, it finally fixes itself 

 upon and usually bores its way into the body of a water-snail 

 or some other mollusk. The tail then drops oft' and the body 

 incloses itself in a cyst. The coronal booklets of the perfect 

 form now appear. It now remains quiescent, unal)le to devel- 

 op further in its present situation, awaiting for some water- 

 bird to swallow the mollusk in which it is imbedded. As soon 

 as this is done and the cyst is set free in the alimentary canal 

 of the bird, further development begins and the complete or 

 Distoma form is assumed. The body elongates and narrows 

 anteriorly, the suckers move nearer the head and the circle of 

 booklets being complete, it attaches itself by these to the walls 

 of the intestine. Such is the strange life history of this intes- 

 tinal worm ; and although that of the liver fiuke nuiy vary 

 in some respects, yet it is doubtless similar in a general sense. 



" The following outline, given in my address before the Illi- 

 nois Wool-growers Association, September 20, 1877, is prob- 

 ably substantially correct : 



' They produce a kind of spore, or egg, but its subsequent 

 progress, so far as it is at present known, presents one of those 

 singular life histories occasionally met with in the lower order 

 of animals. In some way, not well understood, this egg or 

 germ spore makes its way to the external world ; its history 



