116 PROCEEDINGS OF THE ACADEMY OF [Feb., 



4, 5 and 23, pro. 2). The fibrillffi in each layer are parallel to one 

 another, and accurately spaced about l.b p. apart, running diagonally 

 from one side to the other of the proboscis, at an angle of approxi- 

 mately 45 degrees to the horizontal plane. The fibrilke on opposite 

 sides of the same partition, however, are not parallel, but, while inter- 

 secting the horizontal plane at the same angle, the fibrillse of one layer 

 cross those of the other at right angles. Thus it follows that only the 

 fibres of alternating layers are parallel, while those of any two con- 

 secutive layers run at right angles to one another. As a result of this 

 arrangement, in cross sections through the proboscis, in which two 

 layers of fibrillse are included, these latter, being seen simultaneously, 

 create the appearance of a lattice work, which is very regular whenever 

 the section is parallel to the partitions (fig. 30). The spaces between 

 the partitions are filled with a pale and vacuolated cytoplasm, near the 

 centre of which is a much flattened nucleus, with a large karyosome. 

 The greater portion of the proboscis is thus composed of a series of disk- 

 like cells, placed like coins in a pile, and separated by cuticular parti- 

 tions. Near the centre of each is a nucleus, while each of its faces bears 

 a layer of fibrillse. This peculiar and somewhat complex structure 

 demands an explanation, but it must be confessed that I have none 

 to offer, based on my own observations, inasmuch as living material 

 has been inaccessible to me since the necessity of its study in regard 

 to the point in question became apparent. Some of the observations 

 of Korschelt (1882), made on D. apatris, however, bear on this point. 

 This investigator described and figured the two portions of the pro- 

 boscis, but interpreted the striated appearance of the hinder portion 

 as produced by circular muscles. He then proceeds to say: "Die 

 ganze Riissel strekt sich dabei und verlangt sich durch Contraction der 

 Ringmuskeln." In speaking of the degree of extension of the pro- 

 boscis he states that "seine Vorderende bis in die gegend der Augen 

 vorgeschnellt werden. ' ' So great a degree of extension could hardly be 

 accomplished by the action of extrinsic muscles alone. ^loreover, the 

 diagonal fibrillse by their contraction would produce the same me- 

 chanical effect as circular muscles, viz., an extension of the proboscis 

 brought about by a lessening of its diameter. In view of this fact, and 

 of the observations of Korschelt, it may, I think, be permissible to 

 assume, provisionally at least, that the function of the diagonal fibrils 

 of the proboscis is the extension of that organ. 



In addition to the intrinsic musculature the proboscis of D. conklini 

 is also provided with a set of extrinsic muscles. These are eight in 

 number, arranged in four pairs: protractors, flexors, retractors, and 



