THE FRESH-WATER SPONGE. 457 



when the parent dies at the end of the season, they are liberated and pass from its body. 

 They then attach themselves to subaquatic substances, such as vegetables, sticks, stones, 

 etc., and at the beginning of the next season the two discs separate, and out comes the 

 young Cristatella, ready to take upon itself the tasks for which it was created. It 

 often happens that the two disks of the statoblast cling to the young for some time after 

 it has given up its contents, and the little creature carries about the separate halves in 

 a manner that reminds the observer of a bean newly sprouted from the ground and 

 bearing the two halves of the seed which was planted. The Cristatella also produces 

 buds, and in fact, the statoblast is a kind of bud of rather peculiar construction. The 

 disc of the statoblast is brown. 



At fig. F is an example of an interesting polyzoon found in our ponds aad streamlets, 

 adhering to the rootlets of duckweed, and deserving of peculiar interest as being the first 

 species of polyzoa that was detected. The honor of its discovery rests with Trembley, 

 who named it appropriately " Polype a panache," the plume-like group of tentacles 

 being sufficiently large to be seen with the naked eye. In this creature, the place of the 

 external wall or ectocyst, is taken by a soft gelatinous envelope, as may be seen by 

 reference to fig. C, where the creature is represented on an enlarged scale. 



Mr. Allman remarks that in the interior of the Lophopus are often to be seen a vast 

 number of little glittering particles of a pear-like shape, which move about through a 

 series of tubes connected with each other like the capillary vessels of the vertebrates. 

 After much investigation of the subject, he came to the conclusion that they were merely 

 parasitical. 



WITH Plate X. is concluded the history of the polyzoa. At fig. A. is shown a portion 

 of a fresh-water polyzoon called scientifically the Alcyonella fu?igosa, and popularly 

 the Fresh-water Sponge, because when dry it has a very sponge-like aspect. It is found 

 in masses of considerable size, sometimes weighing as much as a pound, adherent to 

 various substances which are constantly beneath the waters in which it lives. It 

 frequently develops itself round the pendent twigs of the weeping-willow and other trees 

 which dip the extremities of their branches into the water. I have seen the timbers 

 of locks quite encrusted with the Alcyonella in many plates, and I have in my pos- 

 session a very fine specimen which I took out of the reservoir at Swindon. 



When carefully removed and placed in fresh water, it gradually develops a kind of 

 white downy appearance over its entire surface, which disappears with the rapidity of 

 magic if a hand is moved quickly over the vessel in which it is lying. This downy 

 appearance is caused by the tentacles which protrude themselves in vast numbers, and 

 instantly retract when the creatures are alarmed. 



At fig. B is given a magnified view of an Alcyonella which has been cut in order to 

 show the shape and arrangement of the tubes, and it will be noticed that two of the 

 tubes have run at right angles with the others. A few statoblasts are also seen occupy- 

 ing the lower parts of the tubes. At fig. C is represented one of the polypes, showing 

 its beautiful plumy tentacles arranged in horseshoe fashion. 



A polyzoon already mentioned, and named Plumatella repens, is seen at fig. D, shown 

 of the natural size. Mr. Allman has made some curious observations on this species and 

 the manner in which the food is passed through the digestive organs. The process is too 

 complicated to be described at length, but is very remarkable, the food being tossed back- 

 wards and forwards in a singular manner, then a portion pinched off by the end of the 

 stomach, which assumes a kind of hour-glass shape, and in fine pushed here and there in 

 so rapid a manner that it soon loses all the appearance which the various particles 

 presented when taken into the stomach, and becomes of a brown color, evidently caused 

 by the secretions of the stomach. 



Even in swallowing its food, the Plumatella displays considerable powers of discrimi- 

 nation, accepting some particles as they pass over the mouth, and rejecting others as 

 unworthy of reception. This is a tolerably common British species, and is plentiful in 

 Ireland. Its usual places of abode are under stones, submerged branches, floating leaves, 

 and similar substances. Sometimes it attains a considerable size, spreading over a 



