GRANTIA 57 



remains attached to the parent body, other buds develop, and thus 

 a colony of attached individuals arises. 



In sexual reproduction, differentiated sex cells, both male and 

 female, arise in the middle layer of the same individual. It will 

 be remembered that an hermaphroditic condition of the same 

 nature as this was noted previously in Volvox, and other examples 

 will be found in the higher types noted later. The mature sex 

 cells, both eggs and sperm, are discharged into the water, and the 

 fertilization which ensues produces a zygote which divides and 

 gradually develops independently into the mature Grantia. In 

 certain fundamental features of embryonic development, Grantia 

 and the Sponges in general are unique. On this account it will 

 be well to defer a description of the metazoan development until 

 Hydra is considered in the next chapter. 



4. Adaptation 



It has previously been emphasized that protoplasm, wherever 

 found, is responsive to environmental stimuli — it is irritable 

 material — and this characteristic is responsible for the power of 

 adaptation which living organisms possess. Now although all 

 protoplasm is irritable it becomes necessary in the higher types of 

 animals to develop a special type of cell, the nerve cell, or neuron, 

 and to combine these into a complex nervous system which 

 functions for the reception of stimuli and for the inauguration of 

 the proper coordinated response. In the Sponges, this stage of 

 nerve cell development has not been attained, and so the stimuli 

 must be received directly by the various cells. 



TEXTBOOK REFERENCES 



Woodruff, pp. 67-71. 



Curtis and Guthrie, pp. 237-243 ; 316-317. 

 Guyer, pp. 133-135; 142; 666-667. 

 Hegner, pp. 86-101. 

 Newman, pp. 153-159. 

 Shull, pp. 54; 84-85; 251. 



GENERAL REFERENCES 



Cresswell. Sponges (Pitman). 



Moore. " The Commercial Sponges and the Sponge Fisheries " (U. S. Bureau 



of Fisheries, 1910). 

 Parker and Haswell. Textbook of Zoology (Macmillan). 

 Sollas. " Porifera," in the Cambridge Natural History (Macmillan). 



