32 REPOKT OX THE PENNATULIDA. 



and are inserted mainly into the walls of the stomach, especially its 

 upper part, and partly into the bases of the tentacles and the parts 

 immediately around the mouth. Below the stomach each retractor 

 muscle (Figs. 10 and 14/)) does not extend over the whole width of the 

 septum, but is confined to its outer half. 



The transverse sections drawn in Figs. 13 and 14 show some 

 further points of importance concerning these muscles. They show, 

 firstly, that the retractor muscles, whicli lie between the mesoderm (.r) 

 and endoderm (//) do not lie on both sides of the septa, but only on 

 one side of each. A more iinportant point, shown clearly in the 

 figures referred to. is that the muscles do not lie on the same side of 

 all the septa. Thus, on the left hand side of Figs. I'd and 14, is a 

 compartment of the body-cavit}', bounded by two mesenteries in 

 which the retractor muscles face away from one another ; while on 

 the right hand side of the figures is one m which the retractor 

 muscles face towards one another. In the intermediate septa, whether 

 above or below the stomach in the figures, the retractor muscles are all 

 on the right hand side of the septa. 



Owing to this arrangement it is seen at once that there is only one 

 possible bisecting plane that will divide the polype longitudinally into 

 two perfectly symmetrical halves, i.e., a plane passing through the 

 middle of both the right hand and the left hand compartments ; or in 

 Plate II., a plane indicated by a horizontal line drawn across the 

 middle of the figures in question. 



This plane of ,^t/mmetnj, as is shown in Figs. 12 and 18, is also 

 the one which passes through the long axis of both the mouth and the 

 opening from the stomach to the body-cavity, and is the plane of 

 bisection adopted in Fig. 10. 



A less important point, shown by the sections in Figs. 13 and 14, is 

 that the longitudinal muscles extend a short distance round the body- 

 wall on either side of the lines of attachment of the septa, forming, 

 by so doing, the system of longitudinal muscles ef the body-wall 

 referred to on a preceding page. 



Besides the large retractor muscles there is in the upper part of 

 the polype, a second much weaker set of muscles crossing the former 

 at right angles, and having an antagonistic action. These protractor 

 muscles (Fig. 10 q) arise from the upper part of the body-wall, and 

 from the calvx, run downwards and inwards in the septa, and are 

 inserted into the mesoderm of the stomach walls. Their action is to 

 pull up the stomach after it has been drawn down by the retractors. 



f. The Mesenterial Filaments. — These, as stated above, are the 

 thickened convoluted free edges of the mesenteries below the stomach 

 (Figs. 10 and 14, ;■ and s). They are of two kinds:— (1.) A set of 

 two (••*), which are much more slender than the others, but much 

 longer, extending to the bottom of the body-cavity : these we shall 

 refer to as the /«»// mesenterittl jUawenlx. (-2 ) A set of six (Figs. 10 and 

 14 r), which are much thicker and more convoluted,but also much shorter, 



