THE SEA-ANEMONE AND SOME ALLIES 257 



ollow tentacles (Fig. 129, 7), accounts for the absence of a 

 circulatory system, which usually serves the purpose of trans- 

 porting oxygen, digested food, and also waste products from 

 part to part in the animal body. Likewise we find no gills 

 or nephridia, for probably all the waste products of metab- 

 olism are sent from the cells into the gastrovascular cavity 

 and ultimately to the outside. The absence of all these spe- 

 cial organs is further indication of a low and simple type of 

 organism. 



There are usually six pairs of principal mesenteries in 

 Metridium marginatum. They are thin, radial partitions 

 extending from the gullet outward and downward to the 

 body-wall. These six pairs are called the primary mesen- 

 teries (Fig. 129, 10, 11). In the arcs between the sets of pri- 

 mary mesenteries there are secondary mesenteries, also in 

 pairs (Fig. 129, 13), arising from the body-wall and extend- 

 ing part way toward the gullet. The tertiary mesenteries 

 (Fig. 129, 14) and the quaternary mesenteries (Fig. 129, 15) 

 extend shorter distances into the gastrovascular cavity, but all 

 unite with the body-wall of the oral and the aboral ends, and 

 in the latter place meet at the center. Occasionally a mesen- 

 tery grows beyond the usual width and becomes attached to 

 the gullet, as in Fig. 129, 12. Every mesentery has a longitu- 

 dinal muscle-band (Fig. 129, 11) on one side near its free or 

 inner edge. In all but two pairs of mesenteries the muscle- 

 bands face each other. The four primary mesenteries which 

 are directed toward the siphonoglyphes have their muscle- 

 bands on the outside. At the oral end there are two circles 

 of openings called ostia (Fig. 129, 8, 9). They perforate all 

 the primary mesenteries and bring the radial chambers into 

 communication at that end. 



At the free edge of the widest mesenteries below the end 

 of the gullet there are to be found coiled masses which are 

 composed chiefly of mesenterial filaments (Fig. 129, 17, 18). 



