26 MUSCULAR SYSTEM. 



(5.) Anterior Farieto-vaginal Muscles. 



These consist of numerous short bands (PI. IV, fig. 4 ; V, figs. 5, 6 ; IX, fig. 7, s), which 

 arise all round from the inner surface of the cndocyst, commencing close to the line of invagi- 

 nation, and extending for some distance backwards. From this origin they pass transversely 

 inwards, and are inserted into the opposed surface of the invaginated endocyst. Their action 

 will dilate the invaginated endocyst, and assist in keeping it permanently inverted. 



\ 

 (6.) Posterior Parieto-vaginal Muscles. 



These consist of several radiating bands (PI. IV, fig. 4 ; V, figs. 5, 6 ; IX, fig. 7, t), longer 

 and stronger than the last, behind which they arise, from the inner surface of the endocyst in 

 a single plain perpendicular to the axis of the cell, and thence passing upwards and inwards, 

 are inserted into the sheath in a plane parallel to that of their origin, and just behind the termi- 

 nation of the anterior parieto-vaginal muscles. Their action steadies the sheath, and regulates 

 its position during the protrusion of the polypide, while they form a fixed plane on which it 

 may roll outwards with the polypide in the act of protrusion.* 



(7.) Vaginal Sphincter. 



The vaginal sphincter is a circular band (PI. V, fig. 6, ti) surrounding the termination of 

 the invaginated endocyst at the point where the latter passes into the tentacular sheath. Though 

 a contraction of the endocyst at this spot, as if occasioned by the action of a powerful sphincter, 

 may be always observed when the polypide is completely retracted, yet the demonstration of 

 an actual muscle is by no means easy. I have, however, convinced myself of the existence 

 of a distinct structure at the place where the contraction occurs, and, though the presence 

 of fibres is but obscurely indicated, I have no hesitation in viewing this structure as a 

 sphincter muscle on which the contraction in question is dependent. The action of the 

 sphincter closes the sheath after the recession of polypide, and thus protects the latter from 

 all annoyance from without. 



* Tliougli I have here des^cribed, under the names of superior and inferior parieto-vaginal muscles, 

 the two sets of bands which extend between the walls of the cell and the permanently invaginated 

 portion of the endocyst, I am not by any means without doubts as to their being really muscular, and 

 their action after all may be that of simple ligaments. From the study of development in these 

 Polyzoa, it appears highly probable, as we shall afterwards see, that the invaginated portion of the 

 endocyst is originally separated from the proper walls of the cell by a process of cliorization, and while 

 this is taking place the bands in question would seem to be drawn out by the act of separation. 

 There is certainly no reason why muscular fibres may not be developed in them, and the condition of 

 the corresponding parts in Paludicella, as well as in certain marine Polyzoa, is altogether in favour 

 of their muscularity ; but they do not always present the appearance of distinct fibrous fascicles, and I 

 have been unable to detect in them the strise, or see them breaking into the discs, which may be 

 observed in the great retractors of the polypide. 



