252 HIGH SCHOOL ZOOLOGY. 



then be cork-like {Siiberites) or friable in consistence (Si^cmgiUa). One 

 genus— C/(0HO — has the singular habit of boring by means of its spicula 

 into limestone and shells. The sponges of commerce, which come chiefly 

 from the ^lediterranean and the Bahamas, have the skeleton of spongin, 

 either entirely free from foreign matter (in the best Turkey sponges — 

 Emponfj'ia olJicinaUs), or else somewhat coarser in texture, from the 

 builtling of siliceous spicules and fragments of sand into the horny 

 fibre (horse-sponges — Hippospongia). Although much variety of form 

 is to be met with in this family, still wider ranges in this respect are to 

 be found in other families — not only massive, but tubular, funnel-shaped, 

 dendritic and encrusting forms being met with. 



The Protozoa. 



12. All the fore"oinor sub-kingdoms have one feature in com- 

 mon, which is not shared by the lowest forms of animal life ; 

 they pass thi-ough certain stages of development consisting of 

 the segmentation of the eg^^ and the arrangement of the result- 

 ing spheres into a blastoderm, which always possesses at least the 

 two primary layers, ectoderm and entoderm. The tissues, also, 

 in all are the result of the further division and differentiation 

 of these spheres ; but in the sub-kingdom Protozoa, to which 

 we now proceed, development does not take place in this way, 

 and even the most highly organized forms are unicellular, the 

 organs of the body being differentiated out of parts of one and 

 the same cell. These important differences are expressed by unit- 

 ing the foregoing sub-kingdoms under the one designation 

 Metazoa. 



1 3. The Protozoa are generally microscopic in size, and, with 

 the exception of some parasitic forms, are confined to the sea and 

 to fresh-water. Four classes are distinguished — the Sarcodina, 

 Sporozoa, Mastigophora, and Infusoria. The two latter are often 

 spoken of as Flagellate and Ciliate Infusoria, on account of their 

 characteristic locomotive organs ; neither flagella nor cilia are 

 present in the first two classes ; indeed the Sporozoa, being par- 

 asitic forms are destitute of any locomotive organs, and, in the 



