36 Proceedings of the Biological Society of Washington. 



in 1 1/12 and about in line with the ventral seta bundles. The spermathecal 

 pores are median and borne near the posterior border of their somites on 

 small papillae. In one specimen there are two, one each on 8 and 9; and 

 in each of the others there is a single one which is on 9. 



Internal Characters. 



A satisfactory account of the internal organs is difficult and in some 

 respects impossible, because of the poor state of preservation of the ma- 

 terial. The alimentary tract is quite similar to that of M. asymmetricus, 

 since it lacks a gizzard and is otherwise rather simple in type. The con- 

 tents are such as might be expected in specimens living among algae. 

 Diatom shells are especially abundant. A pair of transverse blood vessels 

 connect the dorsal and ventral vessels in the posterior part of each of most 

 of the anterior eleven somites. The relations of the vessels posterior 

 to these somites can not be determined. The first nephridia seem to be 

 in the eleventh or twelfth somites. 



Fortunately the reproductive organs are better preserved than are most 

 others and permit a description of most of the important characters. In 

 the specimens sectioned, there are uniformly a pair of spermaries and a 

 pair of spermiducal funnels in each of somites 9 and 10. The funnels 

 of the anterior pair project freely into the posterior part of the somite, 

 and the ducts extend along the septum to the ventral wall and along the 

 latter to the atrium. Each of the funnels of the posterior pair is included 

 in the anterior part of a corresponding sperm sac. The course of each duct 

 from the funnel is first anteriad, out of the sperm sac, and then ventrad 

 along the septum to the body wall. The courses of the two ducts differ 

 from this point, since each of them extends to the atrium which is on one 

 side of the alimentary tract. The duct from the funnel which is on the 

 same side as the atrium follows a rather direct course to the posterior 

 part of the atrium, the "sperm reservoir," while the course of the duct 

 from the other funnel is between the nerve cord and body wall to the 

 atrial side of the worm and then posteriorly to the reservoir. The sperm 

 ducts of the two pairs in their courses along the reservoir are at first sepa- 

 rated slightly from its wall, but gradually approach it and are in contact 

 with it in the posterior half, and finally penetrate the muscular wall and 

 open into the lumen of the reservoir near its end. In one specimen the 

 courses of the ducts along the reservoir are approximately 90° apart, while 

 in another, two of the ducts are closely approximated. 



The single atrium is on the left side in some specimens and on the right 

 side in the type and in others. It is somewhat similar to that described 

 and figured in M. asymmetricus. The term atrium is here used to include 

 all three parts of the single asymmetrical ectal part of the spermiducal 

 apparatus, since it is apparently homologous with one of the paired atria 

 of Rhynchelmis. In the description of M. asymmetricus the term atrium 

 was used in a different way and applied to a small chamber near the 

 spermiducal pore (Smith, 1896, fig. 7, at). No such chamber is developed 

 in M. lacustris. The atrium terminates in a penial organ corresponding 



