460 J. F. ILLINGWORTH, 



posterior wall of the genital cavity also is attached to the mantle. 

 In this way the genital cavity is completely enclosed, except for the 

 oviduct leading from the anterior part of the right lobe. 



Muscular system. The muscles of the shell are attached 

 around its margin and around the apical opening. Those of the 

 margin are more abundant on the sides near the anterior end (Fig. 1 

 Col). These might be homologised with the columellar muscle of 

 Haliotis for example, though in the Fissurellidae this muscle is much 

 smaller and more distributed. There is not the necessity for power- 

 ful shell muscles here as the shell is of minor importance. The muscles 

 near the anterior end have a special blood supply, as we shall see 

 when we consider the vascular system. 



The muscles of the radular supports 

 are of special importance. The supports, 

 cartilaginous in appearance, and usually 

 spoken of as the radula cartilages, are situ- 

 ^i^és ated just behind the mouth (Fig. J). By 

 sectioning we find that they are composed 

 of a network of irregular cells with thick 

 walls, presenting a close resemblance to those 



Fig. J. Dissection to show the muscles of the 

 head cavity. E eye, E^y epipodium, F foot, Pm pro- 

 tractor muscles to anterior wall of head, Pmf protractor 

 muscles to lower lip, Rad.s radular supports, R.m re- 

 tractor muscles. 



of plant tissue, a hollow cell with suspended nucleus. The muscles 

 that move the supports are attached near the posterior end. There 

 are two sets of protractor muscles (Fig. J Fm and Fm')^ one from 

 the posterior part of the support to the front wall of the head {Fm), 

 the other attached below the first on the support, and extending to 

 the lower lip {Fm'). Behind the radula supports and just below the 

 radular tube is the attachment of the superior retractor muscles {Fm). 

 These are fused near their union with the supports, but run back as 

 two separate muscles to the foot, attaching on either side of the 

 pedal ganglia. 



The inferior retractor muscles {Fm) are two small tendinous 

 cords extending from the lower front angle of the radular supports 

 back to the foot muscles, attaching just in front of the insertion of 

 the superior retractors. These inferior retractor muscles are so white 



