64 Illinois State Lahonitorij of Natural Historij, 



respond to the stain would seem to indicate a difference in the 

 chemical or physical properties of the tissues of the two genera, 

 — a difference hardly compatible, the writer thinks, with any 

 very close relation of the worms. 



The muscle fibers in the longitudinal layer of the body- 

 wall are irregularly disposed (Figs. 13, 16, 17), and cross 

 sections show nothing of the double series so characteristic of 

 this layer in Lumbricus. With this exception there seems to 

 be no essential difference between Diplocardia and Lumbricus 

 with respect to the muscular system. The layers of the body- 

 wall have about the same thickness relative to each other in 

 both genera. Measurement of the body-wall beneath the 

 nerve cord in the anterior part of the body of a Diplocardia of 

 medium size gave a diameter of .40 mm., of which the cuticle 

 and hypodermis together equaled .05 mm., the circular muscle 

 layer .09 mm., and the longitudinal muscle layer .26 mm. In 

 the greater part of the wall of the alimentary canal the mus- 

 cular tissue is not very conspicuous. In the gizzard, of coarse, 

 it is greatly in excess of other tissues. (See Fig. 10.) In the 

 rectum, also, the muscular layers become prominent. Measure- 

 ments of the anterior portion of the rectum gave a thickness 

 of .05 mm. each for the epithelium and circular muscle layer, 

 and about .02 mm. for the longitudinal layer. Near the vent 

 there is a still further increase in all the tissues, measurement 

 giving for the epithelium a diameter of .15 mm., for the cir- 

 cular muscle layer .10 mm., and for the longitudinal layer 

 .05 mm. Everywhere the muscle fibers are bound together by 

 connective tissue, which, in the body-wall, forms, in places, 

 layers of some thickness; but probably nothing comparable to 

 the " bundles " of vertebrate muscle exists. 



Cross sections of muscles present a good deal of variation 

 in the size and shape of fibers. Some of this is due to the state 

 of contraction in which the fibers are fixed by reagents ; but 

 there is still variation in size not to be accounted for in this 

 way, and probably indicating a real difference in the size of 

 fibers. Sections may be .008-012 mm. in longer diameter by 

 .004 mm. in shorter diameter. The ribbon-shai)ed fibrils of 

 which the fibers are largely made uj), are ranged in series ex- 

 tending with the longer diameter of the fiber, giving to 



