120 COMPARATIVE ANATOMY CHAP. 



A. Amphineura. 



The musculature of the Chitonidce has neither been sufficiently 

 investigated nor systematically described. According to the figures 

 of various writers on the subject there are (1) a large longitudinal 

 muscle mass on each side above the foot; (2) numerous muscle fibres 

 which run down from the latero-dorsal region and radiate into the 

 sole ; and (3) the special fibres of the foot, which run through it in 

 various directions. The muscle fibres mentioned under (2) no doubt 

 correspond with the shell muscles of the Fissurellidce, etc., and the 

 columellar muscle of other Gastropods. Some of the fibres descending 

 from one side cross those from the opposite side. These crossings are 

 very marked in the median plane between the two pedal nerve 

 cords. 



Among the Solenocjastres, the muscular system of Proneomenia has 

 been the most thoroughly investigated. In connection, no doubt, 

 with the degeneration of the foot and the vermiform development 

 of the body, a kind of dermo-muscular tube has been formed ; its 

 layers, consisting of muscle fibres running in various directions, are 

 very thin in comparison with the thick epidermis. This muscular 

 tube lies immediately under the epidermis. Its outer layer consists 

 of circular muscle fibres, then follows a layer of diagonal fibres, cross- 

 ing each other at right angles, but crossing the circular and longi- 

 tudinal fibres at an angle of 45. The innermost layer consists of 

 longitudinal fibres, and is most strongly developed on the ventral 

 surface on each side of the ventral groove. Groups of fibres are 

 detached from the circular layer on both sides, and converge towards the 

 base of the rudimentary foot, some of them crossing above it. The 

 bundles which arise from the lateral and upper walls of the body run 

 within the septa which separate the consecutive lateral diverticula of 

 the intestinal canal. 



So far as a comparison between these animals and the Chitonidce is possible, the 

 abortion of the foot and vermiform shape of the body being taken into account, and 

 Chitonellus taken as the transition form, it may be assumed that the circular muscle 

 layer, and in particular the groups of fibres converging towards the foot, correspond 

 with the dorso-ventral muscles of Chiton, and the longitudinal layer with their lateral 

 longitudinal muscle masses. 



B. Gastropoda. 



The only important muscle to be considered in this class is the 

 columellar muscle. This muscle is attached inside the shell to the 

 columella, along which it runs on the right side of the visceral dome 

 and along the right edge of the mantle cavity; it then enters the dorsal 

 side of the foot in which it spreads out. The columellar muscle acts 

 as a retractor to withdraw the animal into its shell. 



