<^ 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, 

 oblique fasciculi.'"' 



With 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. Salpa cylindrica has ten or eleven of tliese jrir- 



III. fig. 16. These striae are due lo a zig-zag pli- dies, the anterior of which converge upon the back 



cation, as 1 have satisfied myself from a specimen and are curved from before backwards ; see Cuvier, 



of Salpa zonaria preserved in alcohol. Will has Mem. sur les Thalides. loc. cit. fig. 9 ; and Savig?it/, 



observed the same in the muscles of other inverte- Mem. loc. cit. PI. XXIV. fig. 1. M'ith Salpa tnu- 



brates {Muller''s Arch. 1843, p. 359). The mus- cronata, and maxima, these girdles are blended 



cular fibrillae of Salpa are bound together in prim- together upon the back ; see Meyen, Veber die 



itive riband-like fasciculi which are plicated duiing Salpen, loc. cit. Tab. XXVIII. fig. 6, Tab. XXI.\. 



contraction hke the frill of a shirt ; this is easily fig. 2. Salpa pinnata (cristata) is remarkable 



Been when one of these fasciculi is observed in an for having numerous anastomoses uniting the gir- 



edgewise position. dies upon the sides of the body and presenting a 



'■i See Savif^ny, Mim. &c. PI. V. fig. 1, 2 {Bol- trellis-like aspect ; see Chamisso, De Salpa, fig. 1, 



tenia and Cynthia) ; Delle Cfiiaje, Descriz. &c., G. II., and Cuvier, loc. cit. fig. 1, 2. 



III. p. 23, Tav. LXXXIV. fig. 3, 5 (J'liallusia) ; 1 Several of these muscles do not always arise 



and the Catal. of the physiol. Series, &c., I. PI. V. directly from the shell, but from the visceral sac ; 



(fhallusia). so that here their action is not solely for displacing 



3 Salpa cordiformis and zonaria have from the viscera, but also for the movement of the valves, 



five to seven isolated and equi-distant muscular to which this sac is attached, 

 girdles ; gee Eschricht, loc. cit. Tab. I. UI. 



17 



