MORPHOLOGY OF MELIBE 529 



(2) T he Muscular S y s t e in . 



The muscular system of M . leoiiina is one of the most 

 striking features of the animal. When the skin and the caecal 

 endings of the liver (PI. 27, fig. 3) are removed, the main arrange- 

 ment of the muscles may be seen to be like that of the inter- 

 woven fibres of a basket, the sides of the animal being supported 

 by a network of muscular fibres. One set runs parallel to the 

 median axis from the anterior to the posterior ends, terminating 

 anteriorly in the periphery of the hood. Dorsally these fibres 

 end by branching in the ridge of the back and in the papillae ; 

 posteriorly they end in the ridge and in the base of the papillae 

 of that part of the body ; ventrally they run parallel to the 

 foot, ending anteriorly and posteriorly in the base of the foot ; 

 the last parallel fibres end in the groove of the foot. Another 

 set runs diagonally, also parallel to each other, and ends in 

 fine fibres anteriorly, posteriorly, dorsally, and ventrally. 

 Hartmann (1880: 11) describes the muscles for Tethys 

 fimbria t a as follows: 'Die Oberseite des Kopfsegels und 

 des Eiickens zeigt audi ofters gediipfelte, manchmal wieder 

 weiss gesaumte Schrag- und Querbander. Dies hat bereits 

 G. Cuvier recht gut abgebildet (Mollusques, Tab. VII, Fig. 1).' 

 It is then seen that Tethys and M e 1 i b e resemble each other 

 in the arrangement of the muscles of the body. In Melibe 

 the muscle-fibres are located midway, between the ectoderm 

 and the boundary of the visceral cavity (PI. 31, fig. 30, Mb). 

 Between the ectoderm and the muscles are a great many 

 connective-tissue cells (Ct) and fibres (PI. 31, fig. 34, Pmc), 

 ends of the branching hepatic system (PI. 27, figs. 4, 7, Hejj), 

 and plasma (PI. 31, fig. 31, Sp). The finer structure of the 

 muscles is of the type common to molluscs : a circular arrange- 

 ment of finer fibrils in each individual muscle-cell (PI. 31, 

 fig. 33, My). The beautiful picture, however, presented by 

 a transverse or longitudinal section through a muscle-bundle 

 needs to be commented on a little further. 



Schneider (1908), describing the muscle structure of 

 Chiton siculus, says: ' Ueber die feine Structur der 

 Muskelzellen ist nicht viel auszusagen. Die langen Fasern 



NO. 268 N n 



