272 F. M. Turnhull — Anatomy and Habits of JSTereis virens. 



Muscular System. 



The walls of the body contain two muscular layers, which are 

 well defined. The first is thick on the ventral side of the body 

 (fig. 22, k), but is thinner elsewhere. It forms in each ring a muscular 

 plane of transverse fibres. The second is placed within the jjreceding 

 and is formed of great fascia? which are attached to the interannuiar 

 partitions (fig. 16, b). This layer does not exist on the ventral side, 

 between the bases of the feet. The interannuiar partitions (fig. 16, a) 

 are attached to the intestine, which they hold in place, dividing the 

 general cavity of the body into a series of chambers (fig. 16) ; but as 

 the inner subcutaneous layer does not extend over the ventral floor 

 of the cavity, the chambers open into one another beneath the intes- 

 tine. I have already mentioned the muscles attached to the fleshy 

 knobs, which hold the aciculfe. These muscles (fig. 22), when they 

 contract all together, force the aciculae outward. When difterent ones 

 contract they move the aciculffi to one side. 



The Nereis virens in swimming moves its body laterally, like a 

 snake. It sometimes has also an undulatory movement, up and down. 

 These motions are all produced by the subcutaneous miiscles. The 

 lateral appendages are used as paddles, but their principal use is to 

 push the worm along in its tube, and for crawling. They are greatly 

 aided by the stifle aciculffi, controlled by their special muscles (fig. 

 22), and by the seta\ When burrowing, the proboscis is used to push 

 away the sand in front and is then withdrawn, while the body is moved 

 forward partly by a vermicular motion and partly by the side ap- 

 pendages. 



Alimentary Syste)n. 



The proboscis is a very remarkable organ and constitutes a formid- 

 able weapon. It is divided by M. DeQuatrefages into three regions: 

 the pharyngeal, the dental, and the oesophageal (figs. 16-18). The 

 mouth has already been partially described. The walls of its cavity 

 are connected by several small, delicate muscles, with the walls of the 

 body cavity, as I have shown in figure 16, m and n. The pharyngeal 

 region commences immediately back of the buccal cavity (figs. 16 and 

 20, x), and has two muscular partitions (fig. 16, c). 



The dental region (20 and 16, jo) is very muscular, and is provided 

 with a considerable number of small teeth, or denticles, which are 

 arranged in groups on the anterior, inner surface of its walls. It also 

 has two large and powerful jaws attached by their hollow bases to the 

 muscles of the posterior inner surface. The worm has the power of 

 turning this dental region inside out. 



