NEUROMUSCULAR MECHANISM IN METRIDIUM 435 



After the color has made its way into the cavities of the tentacles, 

 they may be tied off one by one at the base and put under pres- 

 sure till they burst. In no case did the colored fluid issue from 

 the tip of the tentacle, but invariably from an obvious rupture 

 on the side and we, therefore, feel quite convinced that Thorell 

 ('59, p. 15) was correct in declaring the tentacle of Metridium 

 to be without terminal pores. 



The mouth leads into a long oesophagus which reaches well 

 down into the gastrovascular cavity. The siphonoglyphs, one, 

 two, or three, as highly differentiated grooves extend from the 

 lips inward over the whole length of the oesophagus and ter- 

 minate at the inner opening of this organ. The coarse grooves 

 of the hps are continued as a system of finer corrugations in- 

 ward over the whole length of the oesophagus. 



The mesenteries extend into the gastrovascular cavity in 

 pairs from their attachment on the inner face of the column wall 

 and serve to unite the oral and pedal discs. Certain of these, 

 the so-called incomplete mesenteries, extend only a short dis- 

 tance into the gastrovascular space where they terminate with 

 a free edge. Others, the complete mesenteries, reach across 

 the gastrovascular space and connect the column wall with 

 the oesophagus. The complete mesenteries have a free edge 

 only between the deep end of the oesophagus and the pedal 

 disc. 



A pair of complete mesenteries, the directives, connect each 

 siphonoglyph, with the body wall. Each directive mesentery 

 (fig. 1, compare fig. 3) carries on its free edge a mesenteric fila- 

 ment, which begins where the mesentery becomes free from the 

 oesophagus and extends pedally over about half the edge of 

 the mesentery till it merges into a much coiled organ which 

 may be called the mesenteric convolution. At the pedal end 

 of the convolution each mesentery gives rise to a single acontium. 

 Orally the directive mesenteries are pierced by two apertures 

 a large oral stoma (fig. 1, stm.) very near the lip and a smaller 

 marginal stoma (stm'.) just inside the sphincter muscle (Hert- 

 wig, 79-80, p. 522; Carlgren, '93, p. 108). Each directive car- 

 ries on its exocoel face a well differentiated longitudinal muscle , 



