82 DEVELOPMENT OF 



veloped along with them ; but the earher observers con- 

 ceived an erroneous notion of the true nature of these 

 sacs, since they even denied that the present kind enjoy 

 independent existence and vitahty. Siebold for instance 

 says, that the sac is " a pouch closed on all sides, and 

 possessing neither intestine nor mouth, colourless, and 

 exhibiting no sign of life." This is not altogether un- 

 true vv^ith respect to the sacs when they occur in large 

 clumps and masses in the liver, kidneys, or other in- 

 ternal organs, under circumstances when then* great 

 numbers have prevented their free development ; for 

 then, certainly, no sign of motion is perceptible in 

 them, and should any be perceived, it is impossible to 

 distinguish whether it be referrible to the sacs them- 

 selves, or to the Cercaria swarming within them. But, 

 if smaller, that is, younger sacs, or above all, sacs con- 

 taining only a half-developed progeny of Cercaria, and 

 which lie free, or scattered on the surface of the organs, 

 or what is still better, attached to them by one end only, 

 and with the whole body hanging out in the surrounding 

 water passage be observed, an evident twisting and 

 winding, vermicular motion will be perceptible in them. 

 I have even met with solitary sacs on the walls of the 

 water passages, in which a few slow movements were 

 perceptible, which besides the usual rolling or turning 

 on the axis, consisted in a contraction and extension, 

 which at other times has been very seldom observed. In 

 these solitary sacs it was also manifest, that the surface 

 by which they adhered to the organs of the snail, formed 

 a sort of acetabulum, which however had already lost 

 all power of motion, and was scarcely capable of again 

 affixing itself by suction when once detached. 



I have also witnessed something like this in sacculi 

 which had been immersed in luke-warm water, and very 

 carefully separated from the heap or mass which was often 

 deposited as thick as a swarm of bees upon the branch of 

 a tree, around and in the organs of the snail. 



In the smallest sacculi which I could find (fig. I a, b,) 



