136 



COLLEGE ZOOLOGY 



a 



but do not reach the stomodseum; these are secondary mesen~ 

 teries (Fig. 84, ij; Fig. 85, s.m). Tertiary mesenteries (Fig. 

 84, 14; Fig. 85, t.m) and quaternary mesenteries (Fig. 84, 15) 

 lie between the primaries and secondaries. There is considerable 

 variation in the number, position, and size of the mesenteries 

 (Fig. 84, 12). \ 



Each mesentery possesses a longitudinal retractor muscle 

 band (Fig. 84, 11). The bands of the pairs of mesenteries face 



each other except those of the pri- 

 maries opposite the siphonoglyphes. 

 These primaries, which are called 

 directives (Fig. 8$,d), have the muscle 

 bands on their outer surfaces. The 

 edges of the mesenteries below the 

 stomodaeum are provided with mesen- 

 teric filaments having a secretory func- 

 tion. Near the base these filaments 

 bear long, delicate threads called 

 acontia (Fig. 84, if). The acontia 

 are armed with gland cells and 

 nematocysts, and can be protruded 

 from the mouth or through minute 

 pores (cinclides) in the body-wall 

 (Fig. 84, 18). They probably serve as organs of offense and 

 defense. 



Near the edge of the mesenteries lying parallel to the mesen- 

 teric filaments are the gonads (Fig. 84, 16). The animals are 

 dioecious, and the eggs or spermatozoa are shed into the gastro- 

 vascular cavity and pass out through the mouth. The fertilized 

 egg probably develops as in other sea-anemones, forming first 

 a free-swimming planula and then, after attaching itself to some 

 object, assuming the shape and structure of the adult. 



Asexual reproduction is of common occurrence, new anemones 

 being formed by budding or fragmentation at the edge of the 

 basal disc. Longitudinal fission has also been reported 



'-s.m 



■ t m 



■ p.ttt 



Fig. 85. — Cross-section of a 

 sea-anemone showing the ar- 

 rangement of the mesenteries. 

 d, directives ; p.m, primary- 

 mesentery ; s, siphonoglyphe; 

 s.m, secondary mesentery ; 

 t.m, tertiary mesentery. (From 

 Weysse.) 



