ZooL.— Vol. II.] EISEN—OLIGOCHMTA. 95 



Internal Characters. 



Body-wall. — (figs. 11, 17, 20.) In a longitudinal section 

 of the body-wall we may readily observe how in the anterior 

 somites the muscular layers are arranged in a characteristic 

 manner. The general rule in 01igoch£eta is for the two 

 muscular layers to be rather closely superposed, continuing 

 in the same intimate relation throughout their length. In 

 all the anterior somites of Pontoscolex the distance between 

 these two muscular layers varies, even in various parts of 

 the same somite. Thus, in the posterior part of one of these 

 somites, beginning at the center, the two muscular layers 

 are, as usual, placed one on the other, with no prominent 

 space between. In the anterior part of the same somite the 

 two layers are not so placed, there being large open spaces 

 between them, separated only by narrow muscular strands 

 (figs. 9, 17). The most anterior somites are most differen- 

 tiated in this respect, the posterior ones the least so, and 

 between these two extremes there is a series of intermediate 

 grades. Thus, in somites II to VI there are six small open- 

 ings; in VII to IX, five openings: in X, four openings; in 

 XI, three; in XII, two; and in XIII there is but one. It 

 is not claimed that these numbers are always constant, but 

 they were the same in the three specimens which I sec- 

 tioned up longitudinally. In the posterior somites, begin- 

 ning with XIV, the two muscular layers are as intimately 

 superposed as in most other worms. 



We may also note another peculiarity of the epidermis, 

 namely, the absence of unicellular glands in the anterior 

 part of the somites lying furthest anteriorly. Even in the 

 other somites it is found that the posterior part contains 

 more unicellular glands or goblet cells than the anterior 

 part (figs. II, 17). These glands are of two kind, judging 

 from their staining qualities. In almost every longitudinal 

 section there will be found one or two unicellular glands 

 which stain reddish with such stains as toluidine or thionin ; 

 while all the other glands take a bluish stain. The position 

 of the reddish staining glands appears to be quite constant 

 in the posterior part of the somite. 



