GALLOWAY: NON-SEXUAL REPRODUCTION IN DERO VAGA. 131 



zooids takes place, in connection with the action of the circular muscles 

 to be described in detail later, pulls the ectodermal margin downward 

 and backward into connection with the median-dorsal entodermal and 

 dorso-lateral ectodermal elements previously mentioned. The median 

 dorsal fibres are distributed to the prostomium (Plate 5, Fig. 28, mu. d.). 



New circular muscles are formed among the pre-existing fibres as the 

 bud-zone increases in length. The chief special modification occurs in 

 the immediate vicinity of the future separation. In connection with the 

 ectodermic ingrowths contributing to the formation of the mouth, cir- 

 cular fibres are carried inward in such a way as to constitute an invest- 

 ment of the new epithelium which forms the floor of the mouth (Plate 3, 

 Figs. 16, 17, mu. crc'ce., fibres cut crosswise). 



Before separation of the zooids, these circular fibres (Plate 5, Fig. 25, 

 mu. crc'ce.) pass, like the circumoesophageal nerve ring, between the sep- 

 arated strands of the ventral muscle band, a lie for and part of their 

 course within the nerve ring. Another interesting fact concerning the 

 circular muscle fibres of this region is that they do not constitute a com- 

 plete circular band. The ends of the fibres, instead of meeting in the 

 mid-dorsal line, pass forward as well as upward, and are inserted into 

 the sides of the prostomium (Plate 5, Figs. 26, 29, mu. crc'oe.) ; they aid 

 in pulling the prostomium downward into its normal position when the 

 posterior individual becomes detached from the anterior one. The fibres 

 of this semicircular band (Figs. 16, 17, mu.crc^ae.) are coextensive, on 

 the ventral floor of the mouth, with the latero-ventral ectodermal invagi- 

 nations, and thus contribute to the buccal wall. 



The radial fibres from the dermo-muscular sac to the pharynx repre- 

 sent longitudinal fibres, which have been diverted from their ordinary 

 course of growth, and have become attached to the wall of the newly 

 formed pharynx. Similarly, the fibres moving the bristle bundles are 

 apparently derived from the muscle bands in their immediate vicinity. 

 This is clearly true of those moving the setae in the plane parallel with 

 the axis of the body (i. e. the longitudinal fibres). There are also setal 

 muscles in the transverse plane, but I am not certain as to their origin. 



The dissepiments in the undiff'erentiated region of the anterior zooid 

 seem to be formed in a somewhat mechanical way. The cell prolifera- 

 tions of the ectoderm are, from the beginning, discontinuous. They are 

 practically successive pairs of buds or ingrowths, which force before them 

 the portions of the peritomal lining and its connective tissue investment 

 which lie immediately opposite them. Between successive buds, i. e. in 

 the septal planes, the mesodermic elements remain attached to the per- 



