MUSCULAR AND ALLIED SYSTEMS. 101 



divided, and the two halves of each separated to expose 

 the underlying trunk muscles. The most important of 

 these are the retractor capitis (a compound muscle formed 

 by the fusion of paired median and lateral retractors) and 

 the depressor infundibuli ; these two together probably 

 represent the columellar muscle of the gastropod. 



The right tentacle is extended, the left coiled up within 

 its pouch. 



The muscle-fibres show a very marked double diagonal 

 striation due apparently to the spiral arrangement of the 

 fibrils of the contractile cortex. 0. C. A 63. 



Brock, Morphol. Jahrb., Bd. vi. 1880, p. 202 ; Bal- 

 lowitz, Arch. f. Mikr. Anat., Bd. xxxix. 1892, p. 291. 



C.49. A transverse section of one of the arms of a large Cuttle- 

 fish (Onychoteuthis, Lichtenstein, 'Isis von Oken/ A.D. 1818, 

 tab. xix. See also Loligo banksii, No. 166 D. Catal. Nat. 

 Hist. p. 33), showing the complex arrangement of the 

 muscular fibres. 0. 0. 63. Hunterian. 



C. 50. Right half of the muscular body-wall of a Tunicate 

 (Phallusia mammillata) . The muscle-fibres run in all 

 directions over the body, forming an intricate network. A 

 more regular circular arrangement is noticeable around the 

 oral and atrial apertures. 



C. 51. Muscular body- wall of a Tunicate (dona intestinalis) . 

 The muscle-fibres are arranged more regularly in longi- 

 tudinal and circular layers than in I'kullusia. The fibres 

 that run in a longitudinal direction are gathered together 

 into definite bundles. The individual fibres are very long 

 and of the smooth variety. 



Roule, C. R. Acad. Sci., t. xcv. 1882, p. 45. 



C. 52. Asexual form of Salpa africana maxima, showing the 

 muscular system. The trunk-muscles consist of 9 isolated 

 ribbon-like bauds, situated at intervals along the dorsuin 

 transverse to the long axis of the body. They extend on 

 either side as far as the mid-lateral liue. Around the mouth 

 and atrial aperture are several smaller muscles, the most 



