Professor Herdman on the Structure of Sarcodictyon. 45 



inner surface of the invaginated tube I could not determine. 

 Probably there is no communication. 



The eight mesenteries appear to be similar in all respects 

 in this part of the body. Each consists of a plate of 

 mesoderm connecting the mesoderm of the invaginated tube 

 with that of the body wall, and covered on both faces by 

 a layer of endoderm continuous with the endoderm lining 

 the body cavity. Superiorly the mesenteries join the upper 

 wall of the body between the tentacles when the polype is 

 fully extended, while below the gastric tube they are con- 

 tinued down in a modified condition to the base of the body 

 cavity. 



Throughout the upper part of the mesentery the mesodermal 

 plate is thin (PL I., Fig. 16), and homogeneous except on its 

 lateral edge, where there are thin layers of muscular fibres, 

 which appear in transverse sections as rows of small dots 

 (PL III., Fig. 1). These muscles are visible on the faces of the 

 mesentery in surface view as strong longitudinal fibres, with 

 a few oblique ones crossing them (PL II., Fig. 15, m. f I). 



Lower down, in the portion of the mesentery which joins 

 the gastric tube, the mesoderm is thicker and the muscula- 

 ture stronger. The peculiar plaiting by which a thick 

 bundle of muscle may be formed out of a single layer of fibres, 

 as E. Hertwig^ has described in the Actiniaria, is here 

 found well developed. The single row of muscle fibres is 

 bent many times backwards and forwards, so as to form a 

 deeply sinuous line in transverse section (PL III., Figs. 2 and 

 3). The homogeneous mesoderm is developed into shorter 

 and longer ridges and projections, so as to support the 

 sinuosities of the muscular layer lying on it. This forma- 

 tion of a branched figure in transverse sections, with pinnae 

 and sometimes pinnules, is well developed on one face only of 

 each mesentery (PL III., Fig. 3), but is sometimes present 

 to a slight extent on the other face. By this anatomical 

 feature one face of a mesentery may be distinguished from the 

 other, and in Sarcodictyon, just as in the Actiniaria ^ and the 



^ Zoology of the Voyage of H.M.S. "Challenger," Part xv., Report on the 

 Actiniaria, p. 5. 



2 See Hertwig, loc. cit. 



