32 In vertebrata. 



sponge of lakes and rivers, is a familiar form, and each 

 autumn it will be found to display the formation of 

 winter gemmules or free buds. Halichondria (Chali- 

 nula) is the common sponge found on our sea shores, 

 and Sycandra is the compressed white calcareous 

 sponge found pendulous from rocks, or adhering to 

 sea-weed near low-water mark. 



CHAPTER VII. 



SUB-KINGDOM III : CCELENTERATA. CLASS I. 

 HYDROZOA, JELLY-FISHES. 



General Characters of Hydra. The common Hydra 

 (figs. 2 and 1 8), an inhabitant of our stagnant pools, 

 is the type of the third sub-kingdom of animals. This 

 voracious creature rarely exceeds half 

 an inch in length, and possesses a cy- 

 drical body having the mouth at one 

 end and a sucking disk for voluntary 

 attachment at the other. Around the 

 mouth are six, seven, or even ten 

 slender contractile arms, capable of 

 rapid motion, and about as long as 

 the body, or even longer ; these can 

 be seen actively engaged in seizing 

 Hydra a^Ztaca. p rev an d dragging it into the central 



the orange hydra, 1 J 



showing its tenta- mouth (fig. 1 8 T). The body is com- 



clesandreproduc- ' 



tive organs. posed of two membranes, an outer 



and an inner; the former, which is called ectoderm 

 (outer skin), making up the whole outer surface, the 

 latter (called endoderm or inner skin) lining the interior 



