TEREBELLIML 



91 



the edge of the muscle is slightly curved inward. The ventral longitudinal muscle is 

 about the same length, extending from the lower edge of the oblique to a point on each 

 side of the mid-ventral line, where it is wedged between the oblique and the circular 

 coat. A diaphragm occurs between the fourth and fifth segments, and has two 



m. 



"IXC. 



Fig. 146. — Transverse section of the anterior region of Polymnia nebulosa, Mont. 

 Enlarged, bv., branchial vessels ; other letters as before. 



muscular blind sacs. The nerve-cords lie immediately external to the circular muscular 

 coat, protected externally by a thick layer of hypoderm, and anteriorly are generally 

 carried inward by a ventral groove. No neural canal is present. The circulation in the 

 Terebellids consists of a dorsal and a ventral trunk, with a sinus round the gut, and a 



Fig. 147. — Transverse section of the posterior region of a male Polymnia nebulosa. 



heart between the oesophagus and the stomach. A heart-body is present. Though the 

 gills are absent in some, such as Lesena, the plan of the circulation is the same. 



In the posterior region of the body the outline in section (Fig. 147) is more rounded 

 and the dorsal and ventral longitudinal muscles proportionally thicker, even when the 

 perivisceral cavity is filled with the reproductive elements. 



Other structural features of note are the innervation of the tentacles by the anterior 

 border of the ganglion. 



