258 GENERAL ZOOLOGY 



These filaments are thickly set with microscopic nettle-cells, 

 like those shown in Fig. 127. When the animal is dis- 

 turbed greatly, the mesenterial filaments stream out through 

 the mouth and through small invisible openings in the body- 

 wall. It is likely that the most frequent use of the mesen- 

 terial filaments is in defense. 



The egg-cells and the sperm-cells of Metridium are found 

 in separate individuals, and adjacent to the mesenterial fila- 

 ments (Fig. 129, 16). The development of Metridium has not 

 been studied, but in kindred species embryologists have found 

 that the eggs are fertilized while they are still in the mes- 

 enteries. At a certain stage of development the embryo is 

 freed. It passes out through the mouth of the female, swims 

 awhile by cilia, and finally settles and continues life after 

 the manner of the adult. Reproduction takes place probably 

 most frequently by budding (the formation of small individ- 

 uals on the edge of the aboral surface), and, rarely, by longi- 

 tudinal division into halves. 



Wherever the sea-anemone lives, in tide-pools, on piles of 

 wharves, on rubble beaches, or in the deeper waters of bays, 

 it has to contend with whatever unfavorable conditions char- 

 acterize the place. The periodic fall of the tide leaves some 

 individuals exposed to the drying effect of the air. They meet 

 the conditions by contracting the organs of the oral end, and 

 exposing as little surface as possible to the air (Fig. 126). 

 On account of their tough skin, their nettle-cells, and their 

 ability to hold to a fixed support, it is not likely that adult sea- 

 anemones are ever attacked by fishes or other ravenous animals. 

 We cannot tell how many of the pelagic young sea-anemones 

 are lost by being swept upon the beach, or how many are car- 

 ried far out into the ocean, or swallowed by larger animals. 

 Undoubtedly the number is very great. 



The protection insured to the adult sea-anemone by its 

 tough skin and its nettle-cells is considerable, and we find no 



