Professor Herdman on the Structure of Sarcodictyon. 43 



lying freely in the inter-mesenteric chambers. They 

 appeared to be endoderm cells. 



Tubules are given off from the body cavity into the thick 

 mesoderm of the body wall, and these are all lined by 

 endoderm cells continued in from the surface (PL III., Fig. 1, 

 en. cce). The nuclei of these cells form conspicuous dots on 

 the walls of the tubules in sections stained with picro-car- 

 mine. These tubules probably open into the lacunae lying in 

 the mesoderm, and both tubules and lacunae are continued 

 into the stolons, and thus form a communication between 

 the body cavities of the different polypes of the colony. 



In the cavities of the tentacles, the endoderm cells 

 are more regularly cylindrical (PL II., Fig. 14, en), and 

 are probably provided with cilia, since Gosse states ^ that he 

 observed ciliary action in the interior of the tentacles in the 

 living polype. 



The mesoderm of the tentacles is a thin layer formed 

 almost entirely of muscular fibres (PL IL, Fig. 14, mes). 

 Most of these fibres run longitudinally, and are clearly 

 visible in a squeezed or teazed-out tentacle under a high 

 magnification (PL IL, Fig. 13). They run in irregular bundles 

 along the tentacle, and are continued out into the pinnae, up 

 which they may be traced to the tip. There is also a slightly 

 developed layer of circularly running fibres encircling the 

 tentacle transversely, and placed beneath the longitudinal 

 layer (PL IL, Fig. 14, m.f c). 



The ectoderm of the tentacles has apparently no cuticle, 

 and the cells are not squamous but more rounded, cubical, 

 or low columnar in form, with distinct nuclei which stain 

 readily in picro- carmine. The surface of the tentacles 

 is not smooth, but is raised up every here and there into 

 low ridges or projections (see Figs. 13 and 14 on PL IL). 

 These elevations are due to the ectoderm alone, which in such 

 places is two or three layers deep, and has on its surface 

 between the cells numbers of very small nematocysts and 

 palpocils. I did not find nematocysts in the ectoderm of 

 any other part of the body. 



The irregularly stellate lumen of the invaginated tube is 

 1 Loc. ciL, p. 277. 



