394 University of California Publications in Zoology. [VOL. 6 



bulging prominence. These nuclei are larger and more easily 

 seen than those of the other muscle fibres; this is in harmony 

 with the generally primitive and undifferentiated character of 

 this set of muscle fibres. In the outer transverse muscle layer 

 the fibres are longer, very slightly swollen in the middle of their 

 length, where the nucleus lies very closely applied to one side 

 of the fibre. The nucleus also is elongated, rather than round as 

 in the dorso- ventral fibre (pi. 42, fig. 57). In the two inner 

 layers, the inner circular and the inner longitudinal, the nucleus 

 is very hard to distinguish (pi. 42, fig. 56). The fibres are heavy, 

 elongated, tapering gradually toward their extremities. The 

 nucleus is exceedingly slender, flattened against the surface at 

 one side of the fibre, but still within the fibre. In cross-sections 

 of fibres the nucleus is difficult to distinguish, appearing merely 

 as a thickening of the cell- wall; in longitudinal sections it can 

 be made out more easily. Had not the nucleus appeared so un- 

 mistakably in the outer and sagittal muscles, it probably would 

 have been overlooked in these inner fibres. 



In the cuticular muscle fibres there is no trace of a nucleus. 

 The fibres are slender, long, of even diameter. Those of the 

 transverse layer are exceedingly fine, while the longitudinal ones 

 are of ordinary diameter. The great number of processes run- 

 ning from the subcuticular cells, apparently to insert in the cuti- 

 cular musculature, suggests most strongly that some of these 

 cells, at least, are the myoblasts of the cuticular musculature. 

 The fact that elsewhere in the body the nucleus lies in the fibre 

 makes this seem improbable, yet the writer is at a loss to explain 

 the processes and their connection with the musculature in any 

 other way. 



All the fibre bundles are penetrated throughout and en- 

 wrapped by parenchymal threads, with their accompanying 

 nuclei. The latter can always be distinguished from muscle- 

 nuclei by their shape and size, as well as by position. 



It is of interest to note that Salensky (1874) found in Amphi- 

 lina foliacea smooth muscle fibres with laterally attached myo- 

 blast, this being one of the earliest cases in which a nucleus for 

 a muscle fibre was discovered. Furthermore, the description 

 above given of laterally attached nucleus in the dorso- ventral, 



