INFUSORIA. 17 



difficult to classify tliem properly. They are principally made up of 

 irregular, globular, fan-shaped, palmate, branched, cup-shaped, funnel-shaped 

 (often of great size), tubular, leaf-shaped, or ragged forms, which either 

 surround other objects, or rise upon a short pedicle. The cup-shaped 

 S. usitatissima, and the trumpet-shaped S. tubceformis^ are from the 

 American seas. Tlie former is an article of commerce. S. Jistularis {pi. 

 T5, fg. 44) is a large brown species, with fine and very flexible fibres, from 

 the American seas. 



Notwithstanding the investigations of zealous inquirers, polyps have not 

 been found in sponges. A number of observers who have watched their 

 growth frotfi the commencement, first observed the gelatinous substance, 

 and afterwards the filamentous matter in it, which may be regarded as a 

 skeleton. Tlie growth of sponges is quite different from that of corals, as 

 might be expected from the absence of polyps. Some regard the circulation 

 of water through sponges as a kind of breathing process, a view which is 

 inadmissible. With respect to their propagation, but little is satisfactorily 

 known. According to Olivi, small oval bodies are found in the jelly-like 

 mass, especially in autumn, which have been too hastily named eggs. 



Sponges contain calcareous or silicious spicula, in which they resemble 

 certain plants. They may be detected by burning a piece of sponge and 

 placing some of the ashes beneath a microscope. These spicula are 

 uniform in each species ; so that they are useful in affording specific 

 characters. 



The chief peculiarities of marine sponges are also found in the fresh 

 water species, as Sj)ongillce lacustris / S. fluviatilis ; and S. friabilis., 

 which inhabit swarajDS and running streams. The three named species, 

 however, constitute probably but one, at different stages of growth. 



The following genera are placed here, because, according to Milne 

 Edwards, they have no polyps (new edition of Lamarck, vol. 2, p. 208, 

 520, 522). Some of the best authorities place them among plants. 



Penecillus penecillus^ Linn. {pi. 75*, fig. 3Y), resembles a hair pencil, and 

 the stiin has a whitish calcareous crust (American seas). Flabellaria 

 ojnmtia.^ Linn. {fig. 36) (American seas). Acetabulum mediterraneum {fig. 

 41). Corallina officinalis, Linn. {p)l. 75, fig. 38), white, reddish, or green, 

 four inches long. C. rtthens {fig. 35) (Mediterranean). .These delicate 

 bodies were long supposed to partake of the animal nature of the true 

 corals ; but the structure is now considered to be nearer that of the algse. 



Peclicellaria, Miiller {p)l. 75, fig. 4), is a portion of an Echinus. 



Class Ltfusobia. 



These minute animals have .been observed and studied only since the 

 discovery and improvement of the microscope. Several species were 

 known in ancient times, because they occm'red in such numbers as to 

 discolor the water green or red, in the latter case giving it the appearance 



221 



