238 



CIRRATULID^E. 



stain. Such a coat is probably of a highly elastic nature, and, with the thin circular 

 fibres to its inner side, controls the dilatation of the body-cavity. In some species 

 this is a muscular coat throughout. The elastic basement-layer bends inward at 

 the sides, and ceases at the lower border of the dorsal longitudinal muscle near 

 the lateral blood-vessel, its continuation, bounding the ventral half of the body, being 

 considerably thinner. The dorsal longitudinal muscles are widely spread out, though in 

 some species a median gap occurs dorsally, and with the ventral longitudinal encircle 

 the body. The ventral longitudiual are somewhat thicker than the dorsal and pass to 

 the nerve-area, on each side of which superiorly a blood-vessel occurs. The gut has 

 externally a circular layer under the coelomic epithelium, and internally a thick glandular 

 layer of columnar cells. 



In the anterior third of Cirratulus cirratus (Fig. 1 25) a continuous tough basement layer 



orn. 



Fig. 125.— Transverse section through the anterior third of Cirratulus cirratus, O F. M., from Shetland. 



occurs beneath the thick glandular hypoderm with the tough cuticle, and it often separates 

 with the hypoderm from the rest of the tissues. This elastic coat is one of the controlling 

 tissues in dilatation, as in G. tentaculatus, the other being the comparatively thin circular 

 muscular layer beneath. The longitudinal muscular coat is nearly continuous dorsally and 

 ventrally ; the dorsal, being thickest inferiorly in sections, often curve inwards over the 

 oblique, and in the mid-dorsal line, where they are continuous, a slight increase occurs in 

 some preparations. The ventral longitudinal are somewhat thicker and less expanded, 

 and pass to each side of the nerve-cords. They are boldly fasciculated in transverse 

 section. The nerve-cords lie in a hypodermic area outside the circular coat, surrounded by 

 neurilemma, and have the circular coat internally, whilst the oblique muscles pass to the 

 fibres over their summit. In sections of the anterior third the folded proboscis lies over 

 the gut, and between them is the large dorsal vessel. In the anterior region certain 

 definite large flattened pouches contain the ova, but the relation of these to dissepiments, 

 ovisacs, gonoducts, or other structures is unknown. A section of one is represented on the 

 right of the figure. 



In Chaetozone the structure of the body- wall generally agrees with that of Cirratulus, 



