246 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



A thin layer of dorso-ventral muscles (PL III., figs. 6 and 

 8) is found running along each side of an intestinal 

 diverticulum. In some Lineidse a horizontal muscle layer 

 runs between the proboscis sheath and oesophagus in the 

 mouth region, but this layer does not occur in the present 

 species. The muscle fibres in the head are continuous 

 with the outer longitudinal layer. The inner longitudinal 

 and circular layers of the trunk are not continued 

 anteriorly beyond the brain. In the head, however, are 

 developed an inner layer of longitudinal and an outer 

 layer of circular fibres round the rhynchodseum and 

 cephalic blood lacunae (PL I., fig. 2). 



PROBOSCIS AND PROBOSCIS SHEATH. 



The remaining portion of the muscular system is that 

 in connection with the proboscis and its sheath. These 

 two structures commence at the anterior level of the brain, 

 and their relations to one another and to the rhyncho- 

 daeum may best be gathered from a reference to PL II., 

 fig. 7. The proboscis sheath consists of a layer of longi- 

 tudinal muscle fibres, surrounded externally by a layer of 

 circular ones, and internally invested by the flattened 

 epithelium of its contained cavity, the rhynchoccelom. 

 In its retracted state the proboscis lies in the last-named 

 cavity, and its outer surface is covered by the rhyncho- 

 ccelomic epithelium. 



The internal surface of the proboscis [i.e., in the 

 retracted state) is lined by the high glandular probosci- 

 dial epithelium, which is of ectodermal origin, and is 

 continuous with the epithelium lining the rhyncho- 

 dasum. In other words, the cavities of the retracted 

 proboscis and the rhynchodseum are one and the same, 

 whilst that of the rhynchocoelom is entirely closed 

 and separated from the rhynchodseum by the attachment 



