METRIDIUM MARGINATUM MILNE-EDWARDS 47 



therefore invisible. But when a live Metridium is subjected to 

 strong stimuli, long white threads or acontia are suddenly pro- 

 jected through the cinclides to the outside. The transverse 

 folds of the wall are produced by the contraction of longitu- 

 dinal muscle fibres. The mouth of Metridium leads into a gullet 

 or pharynx which almost reaches the foot and opens into the 

 gastro-vascular space or stomach. We have seen that a gullet 

 had been erroneously ascribed to scyphostoma owing to the 

 temporary invagination of the hypostome under the influence 

 of stimuli. In Metridium the invagination is permanent and 

 the gullet is therefore homologous to the hypostome of scy- 

 phostoma. The lining of the gullet is thrown into longitudinal 

 ridges, except in the siphonoglypkes which are ciliated furrows 

 running from the opposite ends of the mouth down the whole 

 length of the gullet. The number of individuals with a single 

 siphonoglyphe is about equal to that with two siphonoglyphes. 

 The gastro-vascular space is subdivided by longitudinal mesen- 

 teries or partitions. Of these, six pairs are attached to the 

 wall and to the gullet and are called complete or primary mesen- 

 teries. They have in their wall longitudinal muscle bands often 

 called muscle-banners. In diglyphic specimens (with two siphon- 

 oglyphs) two pairs of the primary mesenteries are called directive 

 mesenteries. They are attached to the siphonoglyphs and their 

 muscle-banners are directed away from each other. In each of 

 the other four pairs the banners are directed toward each other. 

 In monoglyphic specimens a single pair of directive mesenteries 

 is present and in the remaining five pairs the banners are directed 

 toward each other. The diglyphic type presents therefore two 

 planes of symmetry, intersecting at right angles, while the 

 monoglyphic type is strictly bilateral. The space between the 

 gullet and the body wall is divided by the primary mesenteries 

 into twelve chambers, but the mesenteries being arranged in 

 pairs, the chambers between the pairs are much larger than the 

 chambers formed by the two mesenteries of the same pair. 

 The former are called exocceles, the latter endoccdes. The endo- 



