66 



Xll. MUSCULAR SYSTEM. 



The strong development of the muscles in Cephalodiscus indicates a great amount of 

 contractility. The .system consists almost entirely of longitudinal fibres, which pass from the 

 stalk to the proboscis, interrupted b\- tlie two transverse septa of the body-cavity. The more 

 important muscles of the metasome pass along the anterior surface of the body-wall, in correlation 

 with the origin of the stalk from this surface. The strongest muscles of the collar pass right 

 and left of the mouth, while those of the proboscis diverge from septum '4 and after traversing 

 the anterior body-cavity are mainly inserted into its anterior wall. The muscular fibres probably 

 originate and are inserted into some part of the basement-membrane which lines the body-cavity, 

 important groups of the muscles ending in the transverse septa. 



The muscles of the stalk have already been described (p. 51). Tracing these forwards, 

 it will be found that at the junction of the stalk and body (PI. XI, figs. 132, 128, 127, 126) 

 the closed circle, or rather double crescent, of the stalk-muscles opens out on its posterior side, 

 and the two ends of the series of muscles become reflected on to the anterior wall of the body. 

 The muscle-fibres are still invested by an amount of connective tissue varying with the species, 

 and the ventral mesentery of the metasome fades away in this mass of connective tissue. In 

 passing into the body the muscular layer soon spreads along the adjacent parts of the bod\-- 

 wall (PI. X, fig. 125; PI. XII, fig. 157), thereby distributing the stress exerted by the contraction 

 of the stalk-muscles. Most of the fibres pass in the direction of the mouth, along the anterior 

 wall of the body. Some of them, however, pass in the opposite direction (figs. 51, 52) along 

 the wall of the caecal prolongation of the metasome which contains the loop of the alimentary 

 canal. Immediately on the dorsal .side of the attachment of the stalk (fig. 126) the muscular 

 layer is thick, and is not much broader than the diameter of the stalk. In passing dorsally 

 (fig. 125) the layer broadens out so as to extend along at least a third of the circumference 

 of the frontal section. The layer becomes considerably weaker as it approaches the collar, 

 no doubt owing to the fact that some of the fibres successively insert themselves into the 

 basement-membrane of the body-wall of the metasome. In C. dodecalophns in particular it is 

 very obvious that the basement-membrane which lines the third body-cavity is specially thick 

 in the region of this muscular layer. 



When the dwindled remains of the muscles have arrived near the gill-slits (PI. VI, fig. 69; 

 PI. XII, fig. 157) most of the fibres are left on the part of the body-wall which is anterior to 

 the position of the external gill-pores, though a few of them pass behind these apertures. With 

 the commencement of the collar-region (which in frontal sections is first seen on the ventral 

 side of the mouth) the two coelomic sacs cease to constitute a median mesentery, and become 

 widely separated from one another {^x'g. 124). They are then found as two triangular cavities, 

 nearly filled with longitudinal muscle-fibres, and occupying a position immediately anterior to 

 the gill-slits (figs. 68, 123, 156, d.c:''a.). We are in fact dealing with two antero-ventral horns 

 of the third body-cavity which extend forwards into the collar-region, in much the same way 

 that the dorsal perihaemal cavities extend into the collar in Balanoglossus. The ventral horns 



