670 PROCEEDINGS OF THE ACADEMY OF [DeC, 



rauskelschlauchs durchdringend, die von der eiueu Ringmuskel- 

 schicht kommendeu iiber Kreuz an die andere hinantreten." 



The definition, then, of the inner circular muscle is a layer of 

 circular muscle fibres that encloses the rhynchocoel and anterior 

 intestine, the individual fibres being just like those of the body 

 wall. 



Since there is a common agreement Avith Burger's definition 

 among the three muscle layers under discussion, namely, that in 

 Mlerura alaskensis, that in Zijrjeupolia, fig. 23, i.C.M., and that in 

 Micnira cceca ; and furthermore, since all three layers have muscu- 

 lar crosses between themselves and the circular muscle of the body 

 wall, I can see uo reason why each should not with all propriety 

 be termed an inner circular muscle layer, homologous with that of 

 the Proto- and Mesonemerteans. 



The only difference between the inner circular muscle layer of 

 Zygeupolia and that of the Carinellas and Carinoma is in the 

 amount of surrounding connective tissue, the body " parenchym " 

 of Burger. In Zygeupolia this tissue is present only around the 

 blood vessels, so that the inner circular muscle layer adjoins the 

 inner longitudinal muscle layer, except where the blood vessels 

 intervene. In this particular Zygeupolia resembles Carinina, for 

 in Carinina the inner circular muscle borders directly on the longi- 

 tudinal muscle, and there is no intervening layer of " Leibes- 

 pareuchym." 



The presence of the inner circular muscle layer and the muscular 

 crosses in Zygeupolia and Micrura is important when viewed from 

 a phylogenetic standpoint. This peculiar region of the body, just 

 in front of the middle intestine, is by far the most interesting part 

 of the whole trunk, for it has unaccountably remained in a primi- 

 tive condition, as comparison with other Nemerteans will show. 



Diagram 2 is a representation of the stages, illustrated by 

 living, more primitive genera, through which the inner circular 

 muscle of Zygeupolia and Micrura may have passed. A, represents 

 the most primitive condition, found in Carinella annulata, in 

 which the inner circular muscle is a thin layer of uniform thick- 

 ness, extending throughout the body; in B, Carinella polymorj)ha, 

 the muscle is co-extensive with the body, but is thickened in a 

 certain region ; in C, Carinella linearis, the muscle ends shortly 

 behind the thickened region; in D, Carinoma, the muscle ends 



