§ 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 
from 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,have, 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 Sagitta forms an excep- 
tion. Its muscular fibres are distinctly striated, 
and its whole muscular system consists of a simple 
layer posed 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. 
8 Thus, with Patella and Haliotis. 
4 See Forskal, Icones, &c., Tab. XXXIV. fig. A.; 
Delle Chiaje,y Memor. loc. cit. Tav. XLI. fig. 1, 
and Descriz. loc. cit. Tav. LXIII.-IV.; Quoy and 
Gaimard, in the Ann. d. Sc. Nat. XVI. 1829, Pl. 
Il. fig. 4-6, or in Isis, 1833, Taf. VI. (Pterotra- 
chea and Carinaria) ; and Rang, in Mém. de la 
Soc. d’Hist. Nat. de Paris, loc. cit. p. 375, Pl. IX. 
fig. 1, 10, a. d. (Atlanta). 
6 See Eschricht, loc. cit. Tab. I. fig. 5 (Clio) 
and Van Beneden, Exercises, &c., Fasc. II. Pi. I. 
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, Piscium et Amphibiorum. 
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 Vertumnus 
thetidicola. 
Delle Chiaje, who formerly described them 
under the name of Planaria oceilata, has since 
concurred in the 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 obtain 
nourishment ; see his Memor. loc. cit. I. p. 59, Tav. 
II. fig. 9-15, II. p. 265, III. p,141, Tav. XX XIX. 
fig. 1, and his Descriz. &c. I. p. 37. Although the 
real nature of these appendages was made known 
long ago by Macri (Atti della reale academia delle 
IL. (Cymbulia and Tied It is possibl 
that Thetis uses as natatory organs, beside its 
large cephalic fin, the contractile appendages 
which exist on each side of the back. These last 
* [§ 204.] For histological studies on the 
lar tissue of the Cephalophora, see Lebert and 
Robin (Miller's Arch, 1846, p. 129)and Leydig 
loc. cit. (Siebold and Kélliker’s Zeitsch. II. 1850, 
p. 191). According to the first-menttoned observers, 
the intimate composition of this tissue with species 
they examined (Mytilus edulis, Buccinatum 
nudatum, and Pecten), is very delicate primitive 
fibrillae which are either smooth and uniform, or 
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 (Miiller’s Arch. 
1842, p. 418). 
are finely punctated through their whole length. 
With Paludina, Helic, Bulimus, Carecolla, 
Leydig found the essential element of this tissue 
to consist of a tube, formed hy the fusion of cells 
linearly arranged ; the nuclei o° these cells were 
often visible. 
‘My own observations on J 2tica heros agree 
with those of Leydig — that the essential structure 
is a fibre and not a fibrilla. — Ep. 
