43G Prof. M'Intosli on the 



dorsal muscle are seen from the anterior border of tlie 

 mouth to a little behind the tentacular bases. Moreover, 

 a thin but distinct layer of circular fibres envelops the 

 body-wall — after its complete formation — beneath the hypo- 

 dermic basement-tissue. It is continuous to the posterior 

 end of the worm. 



In the space occurring on each side of the mouth, and 

 which is in connection with the great vascular chamber, 

 are a series of vertical muscular fibi-es, proceeding from the 

 dorsum and attached in the first instance to the upper border 

 of the buccal mucous tissue. They occupy the region 

 between the longitudinal ventral muscle and the latter, and 

 probably act as retractors of the tissue. The fibres soon 

 increase in strength, and occupy the whole of the somewhat 

 triangular area between the longitudinal muscle laterally 

 and a line drawn from its inferior curve to the dorsal median 

 line. The fan-shaped mass of fibres converges to pass the 

 longitudinal muscle, but being connected with the pro- 

 boscis their insertion is not observed so readily ; though in 

 extrusion of the latter organ they are seen to bend over 

 into it and become attached, chiefly at the external aspect. 

 In the latter condition of the proboscis the fibres anteriorly 

 assume, for the most part, a vertical direction, and, as it were, 

 bound laterally the great mass of longitudinal muscular 

 fibres which are placed next the inner border in the extruded 

 organ. Then (proceeding backward) it is noticed that some 

 of the outer vertical fibres, which pass down from the 

 dorsum close by the ventral longitudinal muscle, become 

 attached to the chitinous continuation of its inferior 

 boundary, and thus cease to extend into the extruded 

 proboscis. Further, the fan-shaped mass of fibres is split 

 into two by the intervention of the blood-channel. Many 

 of the fibres still pass into the proboscis, but they soon cease 

 to do so, and are fixed to the chitinous plate above mentioned, 

 in company with the great transverse ventral muscle. The 

 latter is attached to the ventral aspect of the chitinous base- 

 ment-tissue, and therefore probably acts in opposition to 

 the fan-shaped vertical series. From the extensive con- 

 nection of the latter with the dorsal region, and the strength 

 of the transverse ventral muscle, almost the whole body- 

 cavity will be firmly compressed during their contraction 

 and the proboscis expelled. 



The interposition of the blood-channel in the great fan- 

 shaped anterior muscle — already noticed — separates it into 

 a vertical and an oblique divisif)n. The vertical fibres 

 spring from the common origin at the outer border of .the 



