Slructure o/*Magelona. 435 



stretching from a cliitinous origin at the inner surface of tlie 

 liyp;)clerm, at a point over the longitndinal ventral muscles, 

 to a similar attachment on the opposite side. In contraction 

 it usually presents a double fusiform appearance — pointed 

 externally, and narrowed at the middle line under the 

 longitudinal muscles. The tentacular blood-vessels seem to 

 pass between it and the longitudinal ventral in this region. 

 While the external terminations of the muscle immediately 

 become simplified by attachment to the upper end of the 

 chitinous support (which becomes continuous with the 

 dorsal hypodermic basement-tissue) of the longitndinal 

 ventral (at this point lateral) muscle, the central region is 

 rendered complex by the gradual increase of the dorsal 

 longitudinal muscle, which causes the fibres to bend down- 

 wards*. Thus a narrow band of the vertical muscle passes 

 over the outer edge of the median longitudinal, and a strong 

 belt of the dorsal transverse separates them from the blood- 

 vessel beneath. A vertical slip of fibrous tissue passes from 

 the hypoderm to the roof of the chamber for the dorsal 

 vessels, between the longitudinal muscles. Thereafter the 

 latter descend entirely beneath the transverse muscle, and a 

 chitinous band separates the tibres in the middle line. The 

 course of the muscle under the hypodermic basement-tissue 

 and its insertion are similar, and it is pierced by the fibres 

 of the oblique. Further backward the distance between the 

 dor?al origins of the muscle greatly increases, until on each 

 side its divisions are thrust into a dorso-lateral and then 

 a lateral position, while the fibres are correspondingly 

 shortened. Toward the ninth body-segment, while the 

 longitudinal dorsal and ventral muscles decrease in bulk, 

 that just mentioned is much enlarged, stretching on each 

 side as a broad and poAverfnl mass over the whole dorso- 

 lateral and lateral regions. At the ninth bodj'-segment it is 

 still large, and its outer border seems to impinge on the 

 great bristle-wings. The increase in the size of the longi- 

 tudinal dorsal and ventral muscles in the posterior region 

 of the body, and their positions, render this muscle nearly 

 vertical and of considerable length on each side. It extends 

 from the hypodermic basement-tissue at the external border 

 of the dorsal muscle downward to the outer border of the 

 ventral and during the reproductive season is often greatly 

 stretched by the vast increase of the ova. 



In the living animal the median fibres of the transverse 



* Longitudinal sections show tlie fibres of this muscle passing over 

 the longitudinal dorsal very clearly, just before its separation bj- the 

 median chitinous raphe. 



29* 



