§204. 



THE CEPHALOPHORA. 



231 



CHAPTER II. 



MUSCULAR SYSTEM AND ORGANS OF LOCOMOTION. 



§ 204. 



The muscles of the Cephalophora are composed of smooth, primitive 

 bundles, which are easily separated into short oblong fragments, and often 

 have numerous nuclei scattered through their substance.* 



The cutaneous muscular system is highly developed ; it consists of a 

 muscular layer made up of oblique, longitudinal and transverse fibres, 

 which are not divisible into separate muscles, and are intimately united 

 with the skin.*'* Upon the ventral surface, with the Gasteropoda, this 

 cutaneous layer is very thick and forms a long disc, — the foot. The fibres 

 of this foot, by contraction, produce wrinkles which succeed each other 

 fi'om behind forwards in a wave-like manner ; by this means the whole 

 foot glides easily over solid bodies or on the surface of the water.*-' Many 

 Gasteropoda use their foot for a sucker also, and then there are circular, 

 tendinous fibres inwoven between those of the muscle proper.® 



With the Heteropoda, there is, upon the ventral surface, a laterally com- 

 pressed process which has numerous muscular fibres. These animals swim 

 upon their back and use this as an organ of locomotion ; while a sucker, 

 situated upon its borders, is used, it is said, as an organ for attachment. *^^ 

 The Pteropoda, Thetis, and Aplysia,\siNe, upon certain places of their body, 

 wing-like expansions, which are traversed by numerous muscular fibres, 

 and are used as oars for swimming.*'' The horizontal fins which are found 



1 Here again the genus Sa^itta forms an excep- 

 tion. Its muscular fibres are distinctly striated, 

 and its whole muscular system consists of a simple 

 cutaneous layer composed only of longitudinal 

 fibres. 



2 The breadth of this foot varies much according 

 to the species. With Scyllaea, and Tritonia, it 

 forms only a very narrow furrow, with which these 

 animals can embrace marine algae. 



S Thus, with Patella and Haliotis. 



4 See Forskdl, Icones, &c., Tab. XXXIV. fig. A.; 

 Delle Ckiaje, Memor. loc. cit. Tav. XLI. fig. 1, 

 and Descriz. loc. cit. Tav. LXIII.-IV. ; Quay and 

 Gaimard, in the Ann. d. Sc. Nat. XVI. 1829, PI. 

 II. fig. 4-6, or in Isis, 1833, Taf. VI. (Pterotra- 

 chea and Carinaria) ; and Rang, in Mem. de la 

 Soc. d'Hist. Nat. de Paris, loc. cit. p. 375, PI. IX. 

 fig. 1, 10, a. d. (^Atlanta). 



5 See Esckricht, loc. cit. Tab. I. fig. 5 (JClio) 

 and Van Beneden, Exercises, &c., Fasc. II. PI. I. 

 II. {Cymbulia and Tiedemannia). It is possible 

 that Thetis uses as natatory organs, beside its 

 large cephaUc fin, the contractile appendages 

 which exist on each side of the back. These last 



* [ § 204.] For histological studies on the muscu- 

 lar tissue of the Cephalophora, see Lebert and 

 Robin (Miiller''s Arch, 1846, p. 129) and Ley dig: 

 loc. cit. {Siebold and KOlliker's Zeitsch. II. 1850, 

 p. 191). According to the first-mentioned observers, 

 the intimate composition of this tissue with species 

 they examined {Mytilus edu/is, Bttccinatum 

 nndatum, and Pecten), is very delicate primitive 

 Lurillae which are either smooth and uniform, or 



have had various interpretations as to their na- 

 ture, from the ease with which they are detached 

 (see Meckel in his programme ; Additamenta ad 

 historiam Molluscorum, Piseium et Ampliibiorum. 

 Halae, 1832). 



Rudolphi (Synop. Entoz. p. 573), and Otto 

 (Nov. Act. Nat. Cur. XI. p. 294, Tab. XLI. fig. 1, 

 a-f.) have taken them for parasites under the 

 names of Phoenicurus varius, and Vertummis 

 thetidicola. 



Delle Chiaje, who formerly described them 

 under the name of Planaria ocellata, has since 

 concurred in llie opinion of the last two naturalists; 

 but he suggests that they may be the young of The- 

 tis attached to the back of their parents to obtiiiu 

 nourishment ; see his Memor. loc. cit. I. p. 59, Tav. 

 II. fig. 9-15, n. p. 265, III. p. 141, Tav. XXXIX. 

 fig. 1, and his Descriz. &c. II. p. 37. Although the 

 real nature of these appendages was made known 

 long ago by Macri (Atti della reale academia delle 

 Bcienze di Napoli. II. 1778, p. 170, Tav. IV.), yet it 

 is only recently that it has been confirmed by Ve.- 

 rani (Isis, 1842, p. 252) and Krohn (^Muller's Arch. 

 1842, p. 418). 



are finely punctated through their whole length. 

 With Paludina, Helix, Bulimus, CarocoUa, 

 Leydig found the essential element of this tissue 

 to consist of a tube, formed ' • *lie fusion of cells 

 linearly arranged ; the nucl i ■■ these cells were 

 often visible. 



My own observations on / ,ica heros agree 

 with those of Leydig — that tl.o essential structure 

 is a fibre and not a fibrilla. — Ed. 



