MAGELONA, 



219 



band. The direction of the fibres of this great muscular expansion is not straight or 

 vertical, but they curve from before backward as well as from above downward and 

 inward. The foregoing arrangement at the ninth segment thus holds the whole body- 

 cavity in control, and, like the complex muscles of the vertebrate heart, must materially aid 

 the blood-vessels in propelling the blood forward into the anterior region and in keeping it 

 there, during the contraction of the other muscles of the part, by firm closure of the 

 channel of communication. The importance, therefore, of this fan-like muscle on each 

 side in regard to the functions of the proboscis is apparent. 



As soon as the body- wall becomes continuous behind the mouth a series of transverse 

 fibres pass from the raphe at the inner end of each longitudinal ventral muscle quite 



OTTV. 



W-ht 



Fig. 124.— Transverse section of Magelona papillicornis, F. Mtiller, in the anterior third. 



across the body, and thus form a very efficient expulsive system for the proboscis as well 

 as a barrier after extrusion. This ventral transverse muscle is continued for some 

 distance backward as a powerful layer, and then a raphe appears in the mid- ventral line 

 so as to form two muscles, each stretching from the common raphe externally to the 

 ventral median line under the hypoderm, and this throughout the anterior region. In 

 contraction they become almost baccate, especially toward the termination of the region. 

 These muscles diminish immediately, so that in the tenth segment only their form is 

 indicated by the small swellings on the band between the ventral longitudinal muscles. 

 Finally, the chitinoid band which alone represents them separates the ventral vessel from 

 the nerve-cords, and externally gives attachment to the vertical and oblique muscles. 

 This double-bellied condition of the atrophied remnant of the transverse muscle is 

 continued to the posterior end of the worm. 



With the diminution and atrophy of the transverse ventral muscles over the nerve- 

 cords in the posterior region the vertical and oblique also coincide, so that by-and-by only 

 a few slender fasciculi of each remain, stretched from the dorsal raphe in the case of the 



