CH^ETOPTBRID^l. 



117 



has the form of a horse-shoe, the dorsal region being concave, the ventral convex. The 

 vertical interdigitate with the dorsal and ventral mnscles in a most complex manner, 

 special bands of longitudinal muscles occurring between the lateral extensions, the nerve- 

 cord, which is hypodermic, lying in a depression exterior to the insertion of the oblique 

 (nc. Fig. 111). The circular muscular layer cuts off ventrally a narrow longitudinal band 

 on the outer base of the lateral wing, and this layer is greatly thickened dorsally. The 

 alimentary canal is comparatively small and surrounded by a sheath of external longitu- 

 dinal and internal circular fibres. About the eighth segment the ventral hypoderm is con- 

 siderably thickened, forming the so-called ventral scutes in Telepsavus (Claparede). The 

 oblique muscle passes from the outer border of the ventral longitudinal and is attached 

 over the nerve-trunk. A powerful muscle passes from the attachment of the latter to the 

 mid-ventral line (and may represent a continuation of the oblique) where it meets its 

 fellow of the opposite side. Its outer end is over the nerve-trunk in this region. The 

 ventral longitudinal muscles are pushed outward to the wings where they form a compact 

 but rather thin layer along the ventral border of the process altogether beyond the nerve- 

 trunk on each side. The central area for the alimentary canal has a strong inner circular 



Fig. 112. — Ventral edge of the body-wall of Chmtopterus variopedatus, Eenier, showing the ventral 

 • longitudinal muscles clasped by the vertical. 



muscular coat and a thicker external layer of longitudinal fibres through which the various 

 fibres from the vertical muscles pass to the circular layer. Moreover, strong bands pass 

 through the circular layer of the body-wall dorsally from this sheath to the basement- 

 layer under the mid-dorsal ridge and they slightly diverge superiorly. The ventral edge 

 of the sheath has no fibres of this kind. 



Claparede observes that there is a multiplication of both dorsal and ventral muscles 

 in the thoracic region, yet in the middle and posterior regions only a pair of large ventral 

 longitudinal muscles exist, for the dorsal with the exception of a few isolated fibres 

 disappear. The vertical or dissepimental muscles attain great size in the thoracic region 

 and make sub-divisions by passing through the ventral muscles, the vertical grasping the 

 longitudinal fibres in groups, the whole causing the thickness and firmness of the anterior 

 region. 



Toward the end of the anterior region the sheath for the alimentary canal becomes 

 thinner and more free, and the space more capacious. 



The blood is colourless in Ch&toptemis variopedatus. In Telejpsavus costarum a dorsal 

 and a ventral vessel are present, and they are joined by connecting trunks in the form of 

 a reservoir, as in the Ariciidge. No capillaries were seen. A single trunk appears in the 

 tentacles, and Claparede thought it was a blind vessel as in the Spionidse. In Chsetopterus 

 a dorsal and a ventral sinus also exist in the median and posterior regions. The ventral 



