475 



5. Parietal Muscles. — In the walls of the pallial sac, to- 

 wards its anterior extremity, may be seen, under a high magni- 

 fying power and with a carefully adjusted illumination, nume- 

 rous delicate fibres which run transversely round the sac. They 

 are, doubtlessly, muscular, and by their action constrict the 

 sac in a transverse direction, and thus aid in the protrusion 

 of the viscera. I have not succeeded in determining how far 

 back they extend, as the structure of the sac soon becomes 

 concealed under cover of the opaque horny cell. 



6. Superior Parieto-vaginal Muscles. — These consist of 

 numerous short bands which arise all round from the inner 

 surface of the mantle, commencing close to the line of invagina- 

 tion, and extending forsome distance downwards. They thence 

 pass transversely inwards, and are inserted into the opposed 

 surface of the invaginated mantle and sheath. Use, to dilate 

 the sheath, and assist in keeping the upper portion of it per- 

 manently inverted. 



7. Inferior Parieto-vaginal Muscles. — These are a set of 

 about fourteen bands, longer and stronger than the last, below 

 which they arise from the inner surface of the pallial sac, in a 

 plane perpendicular to its axis, and thence passing upwards and 

 inwards are inserted into the sheath in a plane parallel to that 

 of their origin justbelow the termination of the superior parieto- 

 vaginal muscles. Use, to steady the sheath, and regulate its 

 position during the protrusion of the viscera, and to form a 

 fixed plane on which it may roll outwards with the viscera in 

 the act of protrusion. 



8. Vaginal Sphincter. — The vaginal sphincter is a circular 

 band surrounding the termination of the invaginated mantle 

 where it passes into the tentacular sheath. Its use is to close 

 the sheath after the recession of the viscera, and thus protect 

 the latter from all annoyance from without. 



Besides the eight set of muscles now mentioned, fibres 

 may be detected in the walls of the stomach ; but these may 

 more properly be described in connexion with the histological 

 structure of the digestive system. 



