268 F. M. Turnhull — Anatomy and Habits of Nereis virens. 



fifth segment. The other tentacular cirri have more nearly the same 

 length in both sexes. 



The abdominal segments increase in size to about the eighth, and 

 then remain nearly the same for some distance along the middle 

 region, but the posterior rings decrease in size, causing the body to 

 taper until it becomes quite slender. The appendages, also, are longer 

 and broader along the middle region than toward either extremity. 

 On the ventral side of the segments the part continuous with the feet 

 is smooth, l)ut the other parts show the strong transverse muscles 

 (fig. 26), which, by their contraction, lessen the size of the body cavity. 



The lateral appendages or feet of Nereis virens are quite compli- 

 cated and wonderful organs. They are biramous (fig, 12), having 

 two rami, one dorsal (A) and the other ventral (B). The aciculse of 

 each foot arise from a crypt which is attached by shroud-like muscles 

 to the base of the foot. The sette arise from the interior of the 

 two rami. The feet are complicated by the addition of other organs, 

 serving for locomotion, sensation and respiration. 



The respiratory organs, often called liguhne, are moi-e or less flatten- 

 ed lobes with their teguments very thin and filled \\\i\\ a rich vascu- 

 lar netw'ork (figs. 22, s^ 25, 26). The upper ramus has two ligulae, a 

 superior one (fig. 12, h) on its upper, and an inferior one (fig. 12, d) on 

 its lower side. At the base of the superior ligula, on a sort of shoulder 

 of its upper edge, is the dorsal cirrus (fig. 12, a). There are also two 

 setigerous lobes smaller than the liguhe, one on each side of the open- 

 ing through which the sette protrude (fig. 12, c and k). The anterior 

 (c) is longer than the other [k) and is connected with the inferior ligula 

 {d) of the upper ramus. The acicula is attached to the inner walls 

 of the ramus forming a partition, which terminates with the end of 

 the acicula between the lolies c and d, (fig. 12) and generally forms, 

 in the middle and posterior parts of the body, a third and smaller lobe 

 (fig. 12, r'). 



The lower i-anius lacks a superior ligula, but it has an inferior one 

 (fig. 12,^) more rounded and not so broad as the others. In the lower 

 ramus, as in the upper, there is normally only one fascicle of setae, 

 but in the lower ramus it is divided into two clusters by the acicula, 

 which, by its attachment to the inner walls of the ramus, forms a par- 

 tition. Here, as in the u])per ramus, there are two flattened setiger- 

 ous lobes, about equal in length, one on each side of the o])ening for 

 the setae (fig. 12, / and e), and the partition formed by the acicula 

 extends to the extremity of the anterior one, dividing it into two small 

 lobes (fig. 12, e and e). 



