HYATT, OBSERVATIONS ON POLYZOA. 97 



VI. Observations on Poly zoa. Suborder Phylactolcmmata . 

 By Alpheus Hyatt. 



[Continued from Vol. IV, page 228.] 



Muscular System. 



The specialized muscles are arranged in two ways, as 

 circular bands, or longitudinal bundles. The Sphincter, 

 and a new set which I have called the Brachial Contractors, 

 are in the first series. 



The sphincter is very broad in Fredericella and in some 

 species of Plumatella, forming a cup shaped cavity by its 

 contraction when the orifice is closed over the retracted 

 polypide (PI. 7, fig. 6, L, PI. 8, figs. 2, 4, L) . In Pectin- 

 atella and Cristatella the breadth of this muscle is much 

 reduced, and the aperture is closed by the contraction of the 

 edge of the orifice, no cup shaped cavity being formed (PL 

 9, figs. 12, 13, PI. 13, figs. 5, 6, A'"'). 



The brachial contractors are a set of small muscles, 

 ranged on both sides of the interior of the lophophore im- 

 mediately below the junction of the bases of the tentacles 

 (PL 12, fig. 2, L'), and thence they descend on the 

 sides, like the supporting knees of a ship's deck, passing 

 between the bases of the outer and inner tentacular bands. 

 They act directly upon the floors of the arms, which arc 

 more or less drawn up into folds by them, and, when the 

 ends of the arms are depressed, these fcflds become well 

 marked, dividing the cavities by series of ridges rising 

 nearly up to the lophophore. 



The second series comprises all the larger muscles of 

 the body. 



The three sets of retractors , previously described , * are 

 theinstruments by Avhich the principal movements of the 

 polypide are accomplished. These are distinguishable at an 

 early age in the young, and have at all times the local dis- 

 tribution of the posterior attachments which led me to 

 divide them into three sets. Three anterior attachments 

 on either side are sometimes visible when the polypide is 



*In general terms on p. 206. 



COMMUNICATIONS ESSEX INSTITUTE, VOL. V. 14. NOV. 20, 1866. 



