MUSCULAR SYSTEM. 27 



(8.) Parietal Muscles. 



In the description of the histological structure of the endocyst, muscular fibres (PI. V, 

 figs. 5, 6, v) were mentioned as entering into the composition of this tunic, and taking a 

 transverse direction round the cell. These may be designated by the name oi parietal muscles. 

 They may generally be seen very distinctly towards the anterior extremity of the cell, but it is 

 not always easy to determine how far backwards they extend, as in most cases, the internal 

 structures soon become concealed under the increasing opacity of the superjacent tissues. 

 In the more transparent genus Lophojms, they seem to be present throughout the whole 

 endocyst. By their action they evidently constrict the endocyst in a transverse direction, and 

 thus aid in the protrusion of the polypide. A muscular tissue is also very evident in the 

 walls of the stomach ; this has already been described in connection with the histology of the 

 alimentary canal, and need not be further referred to here. 



b. Muscles of Paluclicella. 



The muscular system of Paluclicella differs in some important points from that of the 

 species with bilateral lophophores. The muscles may here be divided into five sets : 



(1.) Retractor Muscle of the Polypide. 



This (PI. X, figs. 3, 4, v) resembles in attachments and use the corresponding muscle in 

 the other species, but is not so distinctly divided into two separate fasciculi. 



(2.) Anterior Parieto-vaginal Muscles. 



These constitute four strong fasciculi (PI. X, figs. 3, 4, s), which, arising from the sides 

 of the cell near the top, are inserted into the opposed surface of the invaginated endocyst. 

 The fibres of each fasciculus are inserted one after another in a straight line, commencing 

 near the line of invagination, and extending for some distance down the invaginated tunic. 

 These four lines of insertion are placed at nearly equal distances from one another, and thus 

 cause the orifice and invaginated tube to assume a regular quadrilateral figure. The corneous 

 ribs already described correspond to the centre of the intervals between the insertion of the 

 muscles. 



Mr. Hancock* enumerates, under the name of superior tube retractors, two small 

 additional fasciculi, which he describes as originating below those just mentioned, and as in- 

 serted also below them into the invaginated tube, their insertion becoming of course superior 



* Loc. cit. 



