Pakkeb. — Metridiuni margiuatum. 



EXPLANATION OF PLATE. 



All figures represent transverse sections of the column of Metridium marginatum 

 Milne-Edwards. In each case all the complete mesenteries have been drawn, and 

 in some instances a few of the incomplete ones. All figures are magnified about 

 1.5 diameters. 



Fig. 1. Diglyphic specimen, showing typical hexamerous arrangement of the 

 complete mesenteries, and a regular group of subordinate mesen- 

 teries in a primary exocoel. 



Fig. 2. Diglyphic specimen, showing a very irregular arrangement of the com- 

 plete mesenteries, one group of which resembles the crowded group 

 of mesenteries found opposite the directive pole in the monoglyphic 

 type. In this crowded group occurs an unpaired mesentery {x or^). 



Fig. 3. Monoglyphic specimen, with five pairs of non-directives, showing the 

 regular arrangement of the subordinate mesenteries about the pair of 

 non-directives opposite the directive pole. 



Fig. 4. Monoglyphic specimen, with six pairs of non-directives, showing the 

 regular arrangement of the subordinate mesenteries about the two 

 pairs of non-directives opposite the directive pole. 



Fig. 5. Monoglyphic specimen, with ten pairs of non-directives, showing the 

 crowding of the non-directives in the region opposite the directive 

 pole. 



Fig. 6. Triglyphic specimen. 



Fig. 7. Monoglyphic specimen, showing pairs of mesenteries in which dorsal as 

 well as ventral components are incomplete. 



Fig. 8. Monoglyphic specimen, showing the union of two primary mesenteries. 



