942 PRINCIPLES OF STRATIGRAPHY 



Animals. 



Phylum I — Protozoa. The Protozoa are unicellular animals 

 either naked or enclosed in a cell membrane. In addition, many 

 rhizopods secrete calcareous or siliceous structures, or, by cementa- 

 tion, form a covering of foreign substances. One or more nuclei 

 are generally present, and reproduction is by fission. The Rhizo- 

 poda include the foraminifcra, which secrete shells of carbonate 

 of lime, or build them by cementing sand grains, etc. The shells 

 have one or more chambers (unilocular or multilocular). If many, 

 they increase in size successively, and are arranged in various ways, 

 including nautilian and spiral coiling. In many forms the surface 

 is pierced by fine pores — the foramina — through which protoplasm 

 is extruded in fine streamers forming the pseudopodia. In size the 

 Foraminifcra shells vary from minute shells to those an inch or 

 more in diameter ( Xummulites). They range from the Cambric to 

 the present with several thousand species. 



The Radiolaria secrete horny or siliceous internal structures, 

 which form a much perforated latticework, ornamented by spines, 

 bosses, etc. They also range from the Cambric to the present. 



Phylum II — Porifera (Sponges). The sponges are aquatic 

 multicellular animals in which the body is penetrated by a complex 

 set of canals, into which water enters, through pores in the outer 

 wall. From the canals are given off, at intervals, digestive sacs, and 

 these finally converge into one or more main canals, with large exter- 

 nal excurrent openings or oscnla. Alodern sponges generally secrete 

 a skeleton of horny substance (chitin) and, in addition, many secrete 

 siliceous or calcareous rods or needles (spicules) which are often 

 compound in form. In many older and some modern forms, these 

 unite into solid structures so that the form of the sponge is pre- 

 served. They abound in all marine formations, from the Cambric 

 to the present. The number of extinct and living species is very 

 great. 



Phylum III — Ccelenterata. The coelenterates have a body 

 composed of two cellular layers, the ectoderm and endoderm, the 

 latter enclosing the coclomic cavity into which the mouth opens. 

 An intermediate non-cellular or imperfectly cellular layer is often 

 present but no true body cavity occurs. The animals (polyps) have 

 a simple body in the Hydrozoa — the mouth generally at the end 

 of a proboscis-like elevation, and surrounded by tentacles. Gen- 

 erally they are compound, many polyps being united by hollow 

 tubes. Special polyps for reproduction (gonopolyps) are com- 

 monly developed, and these often give rise to medusae, or jelly-fish 



