$177. THE ACEPHALA. 193 
But Salpa presents a remarkable exception to this, for here the fibres are 
striated.© 
With the Tunicata, the muscular system is most simple, being limited 
to a subcutaneous layer, which, with the Ascidiae, envelops like a sac the 
body of each individual, and is attached to the skin only at the two open- 
ings of the cavity of the body. It is formed of numerous circular and lon- 
gitudinal interlaced muscles, among which there are, here and there, 
gt fasciculi. 
ith Salpa, this cutaneous muscle consists only of a few isolated bands: 
bound together by a thin, homogeneous membrane. These bands, which vary 
much in number, distance apart, and direction, surround the cavity of the 
body, usually in a belt-like manner. They are sometimes straight, some- 
times curved, and their extremities never meet upon the ventral surface so. 
as to form a complete belt, but terminate loosely, or are blended by anas- 
tomoses with adjoining bands. Around the two openings of the body, they 
form real sphincters, : 
By means of this muscle, the Tunicata can enlarge or diminish the cav- 
ity of the body, and thus cause the necessary renewal of water for nutri- 
tion and respiration, beside ejecting the faeces and products of generation. 
The Salpa, by rhythmical contractions of their body (its anterior superior 
opening, being closed by a membranous valve), eject water through its pos- 
terior opening, and thus are propelled along. 
§ 177. 
With the Bivalvia, the muscular system is much more complicated. Not 
only are muscular fibres scattered through nearly the whole body, but in 
certain points, they are so aggregated as to form distinct isolated muscles. 
The largest of these muscles are the Adductores of the valves. With 
the Lamellibranchia, these consist of a single or a double mass of thiokly- 
set, parallel fibres, the ends of which are inserted at opposite points of the 
two valves, Those species which have two of these muscles are called 
Dimya; here one of these muscles is anterior, and the other, larger, pos- 
terior. With the Monomya, there is one muscle alone; this is large and 
situated near the centre of the valves. 
With Brachiopoda, these muscles are more complicated, there being four 
pairs. Part of these, only, are doubly inserted to the valves,” while the 
rest, which arise from one of the valves, are inserted upon the peduncle. 
1 See Eschricht, Over Salperne, &c., p. 64, Tab. 
TII. fig. 16. These striae are due Lo a zig-zag pli- 
cation, as I have satisfied myself from a specimen 
of Salpa zonaria preserved in alcohol. Will has 
observed the same in the muscles of other inverte- 
brates (Muller’s Arch. 1848, p. 359). The mus- 
cular fibrillae of Sa/pa are bound together in prim- 
itive riband-like fasciculi which are plicated during 
contraction like the frill of a shirt; this is easily 
seen when one of these fasciculi is observed in an 
edgewise position. 
2 See Savigny, Mém., &c. Pl. V. fig. 1, 2 (Bol- 
tenia and Cynthia) ; Delle Chiaje, Desuriz. &c., 
TIL. p. 23, Tay. LXXXIV. fig. 3, 5 (Phallusia) ; 
and the Catal. of the physiol. Series, &c., I. Pl. V. 
(Phallusia). 
8 Salpa cordiformis and zonaria have from 
five to seven isolated and equi-distant muscular 
girdles ; see Eschricht, loc. cit. Tab. I. II. 
i 
Salpa cylindrica has ten or eleven of these gir- 
dles, the anterior of which converge upon the back 
and are curved from before backwards ; see Cuvier,. 
Mem. sur les Thalides. loc. cit. fig. 9 ; and Savigny, 
Mem. loc. cit. Pl. XXIV. fig. 1. With Salpa mu- 
cronata, and maxima, these girdles are blended 
together upon the back ; see Meyen, Ueber die 
Salpen, loc. cit. Tab. XXVIII. fig. 5, Tab. XXIX. 
fig. 2. Salpa pinnata (cristatay is remarkable 
for having numerous anastomoses uniting the gir- 
dles upon the sides of the body and presenting a 
trellis-like aspect ; see Chamisso, De Salpa, fig. 1, 
G. H., and Cuvier, loc. cit. fig. 1, 2. 
1 Several of these muscles do not always arise 
directly from the shell, but from the visceral sac 5 
so that here their action is not solely fur displacing. 
the viscera, but also for the movement of the valves, 
to which this sac is attached. ‘ 
