130 bulletin: museum of comparative zoology. 



Posterior to the great thickening of the inner circular body mus- 

 -cles, tlie sheath is extremely thin and cannot be resolved into definite 

 layers, its walls apparently consisting of a few circular fibers only. 



As stated above, the sheath terminates posteriorly at about the 

 middle of the body. 



Proboscis. — As is the case with many of the other organs of the 

 body, the proboscis of this remarkable worm presents peculiarities 

 •quite unlike those described for any other species. The most im- 

 portant of these is the bulb-like expansion (PI. 5, figs. 47, 49) near 

 the anterior end of the proboscis ; this is followed by a constriction 

 through which a narrow canal leads to the middle chamber which 

 extends through the greater portion of the length of the organ. The 

 histological features and musculature of the two chambers are quite 

 different, as will be described below. A second peculiarity consists 

 of a pair of strong muscular bands (PI. 6, fig. 54) which originate 

 among the tissues of the head near the attachment of the proboscis, 

 but which, instead of entering the proboscis immediately, pass pos- 

 teriorly for some little distance in the rhynchocoel and eventually 

 «nter the outer layers of the proboscis. 



After passing through the thick basement layer of the proboscis, 

 the muscular bands spread out to form the main longitudinal muscu- 

 lar layer of this organ. Anterior to the attachment of these two 

 muscular strands, the proboscis is devoid of all muscles, being made 

 up of a dense layer of homogeneous connective tissue wnth thin epi- 

 thelial coverings both externally and internally (PI. 6, fig. 54). 



The proboscis is attached to the tissues of the head in the mouth 

 region, the attachment sometimes appearing to lie a little anterior to 

 the mouth but occasionally a little farther back, the relative position 

 varying according to the state of contraction of the parts. 



There are three distinct regions or chambers to the proboscis, as 

 mentioned above. The most anterior chamlier has very thin walls 

 anteriorly, which thicken rapidly a little farther back. This cham- 

 ber is very short and ends posteriorly in a bulb-Uke expansion, from 

 which a narrow canal leads back to the middle chamber. Sometimes 

 the bulb-like appearance is very marked ; when but little contracted 

 it is much less conspicuous. A section through the anterior cham- 

 ber (P. 11, figs. 67, 68) shows a very thick inner layer of columnar 

 glandular cells of two kinds, probably differentiated by the nature 

 of their secretions. One variety (gV) is finely granular and stains 



