THE MARINE TURBELLARIA. 



283 



6r 





..•• 



•« •• • 



ffut. 



Fig. 61. Eye-spots of Planocera armata. 

 te. tentacles, br. brain. 



The epidermis is composed of very small cells, crowded with a finely granular secretion. 

 I can find no trace of rhabdites. The basal membrane is thick, and occasionally pierced by 

 processes from gland-cells, lying beneath the integu- 

 ment. The muscle layers consist of a thin, outer, 

 longitudinal layer, followed by two layers of diagonal 

 fibres. These are succeeded by a circular layer 

 which is much thicker on the ventral than on the 

 dorsal side. Lastly on the ventral side only is an 

 inner longitudinal layer. 



The pharyngeal aperture is median. From the 

 main gut some seven pairs of gut branches are given 

 off. These again ramify into numerous smaller 

 branches as they run outwards. 



The small pigmented patches, referred to in 

 the short account of the external characters given 

 above, are found in section to be due to the pres- 

 ence of dark bodies in certain remarkable, dorsal 

 diverticula given off from the gut branches (PI. XV. 



fig. 10). The gut branches themselves are lined with very elongated columnar epithelial cells 

 lying on a delicate basal membrane from which they are fi-equently, in my sections at least, 

 torn away. The epithelium of the diverticula on the other hand, though similar in appearance, 

 is structurally much reduced and obscured by the dark bodies referred to above. 



These dark bodies are of three kinds. Firstly, there are numerous rather large, black, 

 spherical bodies, which are evidently produced in the gut epithelium itself With a high power 

 different stages of the development of these bodies are readily distinguishable. They commence 

 as small, lightly-staining masses, which increase in size, and at the same time take a deeper 

 stain. Of their ultimate fate nothing can be said at present. They are scattered pretty 

 regularly through the gut tissue and probably have but little or no share in giving the diver- 

 ticula their characteristic appearance. Secondly, in and about the diverticula there are small 

 masses of little rod-shaped bodies, probably bacteria. These do not occur generally throughout 

 the gut, but only in the diverticula themselves or in their immediate neighbourhood ; in 

 the latter case they invariably have an appearance of streaming into the diverticula. Lastly, 

 I have found in one or two diverticula only, a quantity of exceedingly fine, granular, pigment- 

 like substances. 



The significance of these bodies and of the diverticula themselves is obscure. The latter 

 may be compared directly with the gut diverticula of such a form as Thysanozoon hrocchii. 

 Diverticula of a precisely similar character occur in a species of Planocera, as yet undescribed, 

 collected by Mr Gardiner at Rotuma, whilst Lang's [9] figure of PI. villosa suggests very 

 strongly that a similar feature occurs in this species, though it is true that Lang appears 

 to regard the pigment patches as merely epidermal. 



Mention should be made here of the fact that von Plehn [10] has figured the occurrence 

 of marginal pores opening from the gut in an undescribed Planoceroid. In the species described 

 next the large spherical bodies of the gut epithelium are present, but there are no diverticula 

 and neither of the other kinds of dark bodies. 



