no AN INTRODUCTION TO THE STUDY OF FOSSILS 



its sixteen tentacles and by their aid pushes into its mouth. 

 The tentacles are covered with numerous nettle-cells (nemato- 

 cysts) by means of which any living prey is paralyzed and thus 

 rendered harmless. Through the mouth, the food enters the 

 large, central or digestive cavity, into which is poured a digestive 

 fluid secreted by some of the smaller endoderm cells lining this 

 cavity. The undigested waste is thrown out again through the 

 mouth, while the digested food is taken up by the bordering 

 cells through osmosis, and thus passed to all the cells, each tak- 

 ing what it needs for repair or growth. 



Respiration is performed by the entire surface of the body, for 

 the covering of the cells of the ectoderm (the surface layer of 

 cells) is so thin that osmotic interchange of the excess carbon 

 dioxid of the animal for the oxygen of the water easily takes 

 place. 



There are many muscular fibers, especially along the inner 

 edge of the ectoderm, the contraction of which enables the 

 animal to shorten its entire body and thus withdraw into its 

 protective cup. Accompanying these muscles are large, much- 

 branched nerve cells in contact wdth one another for the regu- 

 lation of their action. 



Reproduction is both sexual, through the development of 

 spermatozoa and ova, and asexual, by budding. Some of 

 the digested food passes through the base of the polyp into the 

 central canal, and up this to the growing tip, where new buds 

 for the formation of new polyps occur ; some passes into polyps 

 which are completely inclosed in urn-shaped cups (gonothecae) ; 

 these latter, — the reproductive polyps, cannot procure their 

 own food, but give rise to spermatozoa or ova, the union of 

 two of which produce a new colony. Reproductive polyps 

 are present only during the breeding season, from May to 

 September. 



Within the gonotheca develop (by budding) usually only 

 minute jelly-fish, — the medusae; these breaking loose from the 

 parent swim about freely and develop spermatozoa and ova, the 



