692 PROTOZOA. 



posed of them. Below these their numbers decrease. They 

 exist, however, in every formation from the Silurian to the Ter- 

 tiary. In most countries Silicious infusorial shells abound in 

 salt-marshes and the superficial marls which are associated with 

 peat. The fertilizing power of the guano is in part attributable 

 to the silicious shells of infusorial Diatoms with which it is filled. 

 Their remains constitute the Berg-mehl,or Mountain meal, which 

 in Swedish Lapland is, in seasons of scarcity, used, mixed with 

 flour, for sustaining life. With the silicious shields of the Dia- 

 toms are also found the calcareous shields of Foraminifera. 



Prof. Bailey has described a bed of infusorial earth at West 

 Point, N. Y. Thirteen or more similar deposits have been discov- 

 ered in other states, sometimes fifteen feet in thickness. These 

 American fossils are mostly found under banks of peat. The 

 forms of this country are similar in character to those of Europe. 



The true position of Sponges, Spongia, is not easily fixed. 

 Like many of the Polypifera, they have a firm horny or stony 

 skeleton, immersed in a soft gelatinous living mass. If they be- 

 long to the Animal Kingdom, they are at the very lowest point, 

 showing no sensation when pierced, torn, burnt, or acted on by 

 acids ; so that in respect to sensitiveness, they are surpassed by 

 some kinds of plants. The species of Spongia are very numer- 

 ous, one hundred and fifty having been described by Lamarck. 

 Their forms are also exceedingly various, (see Chart and Plate 

 XVIII. fig. 23a, b, c.) They are mostly marine, though some 

 are found in fresh water. The best known species and the one 

 seen in shops, S. officinahs, is found attached to rocks in the 

 Mediterranean, and gathered by divers. The cup-shaped sponge, 

 S. usitissima, is found in American seas. 



Agassiz, in his recent work, expresses the decided opinion that 

 the division of the Animal Kingdom called PROTOZOA, differing 

 from all other animals in producing no eggs, does not exist in 

 nature ; and that the beings which have been referred to it, 

 might be divided and scattered, partly among plants and partly 

 among animals. It would however be premature to suppress the 

 entire class until further results have been attained. 



What is the 4th and last Sub-Kingdom ? What does it include ? What 

 is the 9th branch of Zoology ? How is this term derived ? Is the radiated 

 form equally manifest in all ? Into how many classes are the Radiates di- 

 vided? Name the first. Give the general characteristics of the Class. 

 What is the 1st Order? What are its leading characters? Why have the 

 animals been called Sea-cucumbers ? What does Com. Wilkes say of them ? 

 How large are they? What is the 2nd Order? Give its characters. How 

 do the forms vary? What is said of the Cake and Heart Urchins ? What 



