130 bulletin: museum of comparative zoology. 



spread out and attached to the lateral walls of the pavilion in the ante- 

 rior zooid, and to the prostomium in the posterior individual. The dorsal 

 and ventral muscle bands, on the contrary, are both split, it will be 

 remembered, by ectodermic invasions which penetrate between their 

 fibres. Two symmetrically located series of ingrowths penetrate the ven- 

 tral muscle, so that this band throughout the bud zone is made to consist 

 of a median (wm. v.) and two lateral portions (mu. v'.). During the in- 

 growth of the indifferent ectoderm tissue, the lateral parts are pushed in- 

 ward much more than the median, and in the posterior zooid they come 

 to lie inside the circle of the newly formed pharyngeal nerve ring (Plate 

 1, Fig. 6 ; Plate 4, Fig. 21, mu. v^). These lateral fibres of the ventral 

 band, on account of their deeper position, are less interfer^d with by the 

 formation of the ventral groove than is the median strand. Asa result, 

 they constitute the muscular connection on the ventral side of the zooids 

 which persists longest. Thus the mid-ventral strand is sooner free to 

 form new attachments in connection with the mouth and head of the 

 new individual ; its fibres are in fact seen to be distributed to the 

 lower lip, to the sides of the mouth, and to the pharynx, persisting as 

 the principal ventral muscle. The lateral portion of the ventral band, 

 which passes within the nerve ring, becomes attached to the deeper por- 

 tion of the ectodermal constituent of the ventral surface of the mouth 

 (Plate 5, Fig. 26, mu. v'.). 



In the budding zone of the anterior individual the old median band is 

 superficial even to the newly forming circular muscle fibres ; the baud 

 becomes broken up into smaller groups of fibres by the penetration of 

 ectodermal elements. The deeper lateral strands of the ventral muscle 

 band become more conspicuous, and a new median constituent is formed 

 between them; in this way is reproduced the typical single ventral mus- 

 cle. Anterior and posterior to the region of ectodermal ingrowth the 

 strands merge into a normally continuous sheet. 



The dorsal muscle band is likewise separated into a median and lateral 

 portions by the development of the nerve ring, which passes exterior to 

 the extreme lateral portions of the band (Plate 2, Fig. 10, mu. d'.)y 

 but interior to the dorsal (median) part of it. Thus, while the zooids 

 remain connected, the brain is supported by a sling, as it were, com- 

 posed of tlio lateral strands of the dorsal muscle bands (Plate 2, Fig. 10 ; 

 Plate 3, Figs. 16, 17, mu d'.). In this position, some of the fibres become 

 attached to the brain capsule (Plate 5, Fig. 28) ; others remain attached 

 to the dermo-muscular wall at the anterior dorsal boundary of the pos- 

 terior zooid. The contraction of the latter fibres, when separation of the 



