278 GENERAL ZOOLOGY 



a large space which extends radially between mesenteries 

 (partitions) (Fig. 146, 12, 13) to the body wall. 



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. 146, 10, 11). In the arcs between the sets of pri- 

 mary mesenteries there are secondary mesenteries, also in 

 pairs (Fig. 146, 13), arising from the body wall and extend- 

 ing part way toward the gullet. The tertiary mesenteries 

 (Fig. 146, 14) and the quaternary mesenteries (Fig. 146, 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 

 mesentery grows beyond the usual width and becomes 

 attached to the gullet, as in Fig. 146, 12. 



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. 146, 17,18). 

 These filaments are thickly set with microscopic nettle cells, 

 like those shown in Fig. 141. When the animal is disturbed 

 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 for defense. 



The Coral Polyp. The only coral polyp of the North 

 Atlantic coast is Astran'gia da'nse (Fig. 147). It occurs in 

 colonies of many individuals and incrusts rocks or pebbles 

 in shallow water near the shore line from Massachusetts to 

 North Carolina. 



A polyp of Astrangia is about one eighth of an inch in 

 diameter and of variable length. It bears a very obvious 

 resemblance to a sea anemone. The columnar form, the flat- 

 tened aboral surface resting on a support, and the oral end 



