LINEUS. 247 



of the proboscis. When, as sometimes happens, the 

 proboscis is violently extruded and broken off at its attach- 

 ment the rhynchocoelom and rhynchodaeum form a con- 

 tinuous cavity until the proboscis is regenerated. The 

 rhynchocoelom contains a colourless corpusculated fluid. 



At its commencement between the rhynchocoelom and 

 rhynchodaeum the proboscis consists of a layer of longi- 

 tudinal muscle fibres covered externally by rhynchocoe- 

 lomic and internally by proboscidial epithelium (PI. I., 

 fig. 4, i?.). Just beneath the last layer are the two probos- 

 cidial nerves. Further back the proboscis becomes con- 

 siderably thicker, and a layer of circular muscles makes 

 its appearance between the longitudinal muscles and the 

 proboscidial epithelium. From this circular layer at two 

 opposite poles, as seen in transverse section, fibres pass 

 through the longitudinal layer to the basement membrane 

 just beneath the rhynchocoelomic epithelium, crossing one 

 another in two directions (PL II., fig. 6, mcr.). In this 

 way are formed the so-called muscle crosses characteristic 

 of the Lineid proboscis. The crossing is more apparent 

 on one side than on the other. In this region also the 

 proboscidial epithelium is thrown up into papillae, and is 

 highly glandular, whilst just beneath it there is a com- 

 plete investing nervous layer which has been formed from 

 the two proboscidial nerves. Still further back (PI. II., 

 fig. 5), the nervous layer and the circular muscles 

 disappear, so that, except for the absence of the two pro- 

 boscidial nerves, the appearance of a section taken in this 

 region is somewhat similar to that of one taken near the 

 attachment of the organ (ef., PL I., fig. 6). 



The proboscis is attached by its hinder end to the dorsal 

 wall of the proboscis sheath about one-third of the length 

 of the animal from the anterior end. This is effected by 

 the longitudinal muscles of the proboscis being continued 



