48 MORPHOLOGY OP 



Mesoblastic Structures. 



Muscles. — The muscle-fibres of the proboscis are not 

 gathered into bundles. They consist of circular, radial, and 

 longitudinal fibres. The circular fibres are few in number, and 

 chiefly occur in the external parts of the middle third of the 

 proboscis. 



The radial fibres are very few in B. Kowalevskii, but in 

 B. salmoneus and B. Robinii they are commouj and have 

 a very characteristic appearance {v. fig. 94, a). Their peri- 

 pheral ends are very long and fine, occasionally branching. 

 Their central ends taper suddenly from a thick part containing 

 a nucleus to a very fine fibre. These fibres are always plain 

 fibres. Probably the peripheral ends are inserted into the 

 skin, and the central end into the meshes of connective tissue 

 which permeate the body cavity {v. fig. 79). 



The longitudinal fibres of B. Kowalevskii are arranged in 

 concentric rings, and united to each other by a peculiar con- 

 nective tissue, which contains stellate cells with large nuclei. 

 These concentric rings seem to be more numerous in old than 

 in young animals, reaching the observed maximum of eight. 

 This concentric arrangement is not a distinct feature until 

 adult life is nearly reached. These fibres appear in section to 

 have the same structure as those shown in fig, 94, b, which is 

 taken fi'om B. Robinii. The muscles of B. Kowalevskii 

 were unfortunately not examined in the fresh state. 



In B. minutus the longitudinal muscles do not form such 

 definite concentric rings as in B. Kowalevskii, but all the 

 mesoblastic tissues filling the proboscis cavity are broken up in 

 preserved specimens into radial segments. This is not the case 

 in living B. Robinii, and hence is probably due to reagents 

 in B. minutus; as, however, I have never had an opportunity 

 of seeing the latter in the fresh state this cannot be aSirmed, 



In passing inwards from the outside to the centre of the 

 proboscis the structures are thus arranged : 



