1901.] NATURAL SCIENCES OF PHILADELPHIA. 677 



origin of the iauer circular muscle layers iu the Heterouemerteans : 

 (1) the more or less delicate layer of circular muscle fibres encir- 

 cling the alimentary tract in the oesophageal region of Zygeiqyolia 

 and Micrura is a primitive structure, the remnant of a once 

 extensive layer; (2) further investigation will probably demon- 

 strate the presence of similar primitive remnants of an inner 

 circular muscle layer in other Heterouemertean genera; (3) the 

 *' oesophageal muscle " fibres that frequently encircle the alimen- 

 tary tract in the higher Lineidse are continuations of deflected 

 dorso-ventral muscle fibres. They are not primitive but of more 

 recent structure, secondarily derived from the inner circular mus- 

 cle layer by a breaking down of the latter into groups of dorso- 

 ventral fibres. 



The inner circular muscle layer ends with its great thickening 

 just in front of the beginning of the middle intestine, so that 

 posterior to this region the body musculature consists of the usual 

 three layers — outer longitudinal, circular and inner longitudinal 

 (see figs. 24, 25, 26) — which are very thin in the region of the 

 gonads. 



Histology of Muscular System. — Very little neel be said in regard 

 to the histology of the muscle. Each muscle fibre, as Burger first 

 demonstrated in 1890, is a single cell. The nuclei are long and 

 stain deeply. 



In Plate XL, fig. 13 is shown a portion of the circular muscle 

 layer from the posterior end of the body, drawn from a section 

 stained with iron-haematoxylin. At regular intervals, dark areas, 

 contr., are seen, alternating with light areas. The dark portions 

 represent the most contracted part of the layer, the light a region 

 of less contraction, or of rest. In the middle of the light areas, 

 very faint indications may be seen of a smaller region of contrac- 

 tion. The contraction caused by fixation has evidently occurred in 

 waves, as the regular intervals show. These contracted areas were 

 first noticed in the jS^emerteans by Hubrecht (1887). 



Wagener (1863) describes "striated" muscle from a Nemer- 

 tean, the genus of which is not mentioned. He says the striated 

 areas (*' Querstreifungen ") alternate with thinner, lighter parts, 

 and that the cross-striping merges into a non-striated portion. 

 A fibre separated from the bundle has a series of swellmgs 

 ( " Ansehwellungen " ) on its surface. Wagener's Fig, 1, Taf. 



