THE MARINE TURBELLARIA. 



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brown chromatophores, about 12 in each row. In addition to these there is a single small 

 chromatophore on either side of the brain (see PI. XIV. fig. 8). Under surface entirely prim- 

 rose-yellow. 



A number of minute eyes are scattered round the anterior margin of the body. There 

 are also two groups lying over the brain. These form a V-shaped cluster; the apex of the V 

 is directed forwards, and each of its limbs, narrowing from behind forwards, is made up of 

 some 15 eye-spots, which are rather larger than those of the margin'. 



The margin is not folded. In shape this species approaches P. dohrni, Lang, and its 

 structure is closely similar to that of P. siphnnculus (Delle Chiaji), fully described by Lang. 

 It differs however sufficiently from both and from other species admitted by Lang in colour 

 and in details of the arrangement of the eyes. 



It was necessary to examine the specimen by serial sections. I append some details of 

 its anatomy : — 



Body-wall. The epidermis consists of a columnar ciliated epithelium, containing but few 

 rhabdites. It rests on a moderately thick basal membrane. The musculature of the body-wall 

 is more strongly developed on the ventral than on the dorsal side. Immediately within the 

 basal membrane lies a thin layer of longitudinal muscle-fibres, next to these circular fibres 

 followed by diagonal fibres. Lastly, on the ventral side only is an inner longitudinal layer. 



Pharynx. The pharynx is of course tubular as throughout the family. My sections were 

 only cut a little further forward than the proximal end of the pharynx, and at this end its 

 lumen is triradiate in transverse section (Fig. 70, A), This triradial appearance occurs in other 

 species of the genus). It shows some differences in detail from the proboscis of P. siphunculus 

 described by Lang [9]. 



The outer and inner walls are lined with extremely flattened pharyngeal epithelium (Fig. 

 70, B, 0. e. and i. e.). A large number of radial fibres {ra. m.) extend right across the wall from 



Fig. 70. Transverse section through the pharynx of Prosthioslomum elegans. 

 A. An entire section showing the triradiate lumen. B. Part of the same more highly magnified. 



For explanation of lettering see text. 



the inner to the outer epithelium. Immediately within the outer epithelium (o. e.) is a narrow 

 layer of longitudinal muscle-fibres (e. I.), succeeded by a much wider circular layer (e. c). In 

 P. siphunculus this circular layer is succeeded by a longitudinal layer, which Lang calls the 

 retractor muscle layer. This layer is present in P. elegans, but is only poorly developed 

 (m. I.). Amongst the fibres composing it occur a certain number of ganglion cells (g.). Below 



' In fig. 8 the marginal eyes are drawn proportionately too large. 



