Professor Herdman on the Structure of Sarcodictyon. 39 



the result being that the eight inter-mesenteric spaces are 

 lined throughout by endoderm. Each inter-mesenteric space 

 is continued into the cavity of a tentacle. 



A transverse section through the body further down, at about 

 the level of the retracted mouth (see PL I., Fig. 18), shows 

 externally the body wall, while from its inner surface the eight 

 mesenteries project towards the centre of the section. After 

 running some distance inwards, they become complicated and 

 split into two lateral halves, which join the side walls of the 

 neighbouring tentacles (PL L, Fig. 18, tn), seen cut at various 

 angles, and lying retracted in the inter-mesenteric chambers. 

 The open central space in the section is the region immedi- 

 ately above the mouth opening. Fig. 19 on PL I. represents a 

 section cut obliquely through the lower end of the invaginated 

 tube, so as to show the passage from the one condition to the 

 other. The upper half of the section shows the invaginated 

 tube and the simple attached mesenteries, while the lower 

 half shows the thickened and complicated mass formed by 

 the retracted tentacles lying in the inter-mesenteric chambers. 



A longitudinal section through one of the polypes in a 

 retracted condition shows the body wall turned in at the 

 top of the section, to continue the non-retractile into the' 

 retractile portion (PL I., Fig. 13). The retractile or invagi- 

 nated portion (i. t) is found to be folded upon itself, so as to 

 form a sac-like involution on each side of the section. The 

 true mouth opening is shown at m, while the lower end of 

 i. t. indicates the upper edge of the side wall of the body in 

 the completely expanded condition. The region between the 

 dotted lines from tn. is one of the tentacles cut in section ; 

 its tip is seen opposite tn. From this section it is obvious 

 that, when retracted, the tentacles are received into the 

 inter-mesenteric chambers, and are partially but not com- 

 pletely invaginated, the invagination extending for roughly 

 about half of their length, while the terminal portion lies 

 within the cavity formed by the invaginated portion. 



The mouth leads into the gastric tube, which is found in such 

 a longitudinal section to have its walls thrown into a number 

 of strongly-marked horizontal folds, the result being that in 

 a transverse section of such a contracted polype the wall of 



