148 BULLETIN OF THE 



pass. The boundaries of the epithelial cells extend in this case also 

 only to within a short distance of the cuticula ; they cannot ordinarily 

 be traced up to it, except, as in the case of the dorsal line, in material 

 preserved in Tlemming's mixture. The entire ventral line is separated 

 from the body cavity dorsally, and from the muscularis on both sides, by 

 a basement membrane. 



In the adult female the lines do not exceed 8 /* in width, and are con- 

 sequently difficult to study, but I think the same elements can be seen, 

 though not so clearly as in the case of the male. 



Biirger ('91, p. 636) believed the collection of long cells at the apex 

 of the head to be a part of the dorsal line. I can find no special con- 

 nection between the two regions, and no striking similarity in structure. 

 The shallow groove which he believed characteristic of the external 

 surface of the dorsal line is not present in the living animal. It is 

 undoubtedly the effect of collapse, since it is found only in preserved 

 specimens. I did not find the large cells which he says occur at regular 

 intervals in the dorsal line. Perhaps they are found only in individuals 

 of a certain age, or they may be connected with the formation of the 

 hairs. Proof of the existence of a columnar epithelium, which he con- 

 jectured to be present, has been given above, 



c. Muscular Layer. 



In cross sections of the body the muscular layer presents two sharply 

 marked portions, a peripheral, radially striated zone (Plate I. Fig. 11) 

 and a deep protoplasmic region. Along the line of union of the two 

 lies a double or triple row of thickly crowded nuclei, and in some 

 regions, or under certain conditions, other nuclei are found scattered 

 through the protoplasmic portion. The relative thickness of the two 

 zones varies greatly. In the most of the specimens which I cut they 

 were of nearly equal breadth, but in some the protoplasmic zone was 

 more than twice as wide as the striated portion, and in other cases not 

 half so wide. These conditions are represented somewhat diagram- 

 matically in a number of transverse sections (Plate II. Figs. 23 to 26), 

 which are taken from different individuals. 



The general meaning of the two zones is at once apparent. The 

 striated portion is made up of the contractile fibres of the muscle cells ; 

 the protoplasmic area represents the non-diflerentiated portions of the 

 same cells together with certain other elements. I met with indifferent 

 success in attempting to make macerations of this region and I am not 

 able to affirm definitely what proportion of the protoplasmic zone is 



