THE ACALEPHAE. $$ 57, 58. 
62 
CHAPTER II. 
MUSCULAR SYSTEM AND ORGANS OF LOCOMOTION. 
§ 57. 
The Acalephae have a distinct muscular system. Their contractile sub- 
stance is composed of a net-work of elongated, slender filaments and 
bands; these, in the utriculoid species, are arranged in a longitudinal and 
annular manner, but in those of a discoid and campanulate form they are 
disposed in a circular and radiate manner. coal 
In the extremely irritable tentacles and tactile filaments, the longitudi 
nal fibres abound.” 
Each fibre is smooth when relaxed, but during contraction appears trans- 
versely wavy and plicated. 
§ 58. 
The contractile and aérial natatory vesicles, which are found in the Phy- 
sophoridae,® and the movable lamellae of the Ctenophora, may well be 
regarded as accessory organs of locomotion. These last, which are arranged 
in rows upon the sides of the animal, and which by some anatomists have 
been regarded as respiratory organs, are not simple cutaneous lobes, but are 
composed of very long cilia closely united together, and the motion of which 
is voluntary with the animal.” 
I Will (loc. cit. p. 48, Taf. I. fig. 11) has observed 
in the contractile exer of the Eucharis, 
not only circular fibres and numerous longitudinal 
muscles, but large transversely-flattened ones, 
which were bound together by oblique bands. 
2 Will, loc. cit. p. 47, 63, Taf. I. fig.18, Accord- 
ing to Wagner (Ueber den Bau, &c.; and Icon. 
zoot. Tab, XXXIII. fig. 30), the muscles of the 
Discophora have always the transverse striae. 
The cartilaginous natatory pieces of the Siphon- 
ophora play a completely passive part in the act 
of | ti The swimming is exclusively per- 
formed by the energetic contractions of the mus- 
cular membrane which lines their cavity, con- 
stituting, therefore, a true natatory sac. See Sars 
Faun. littural. Norveg. p. 42.* 
1 Lately, it has been’ doubted if the Physophor- 
idae can sink and rise in the sea by means of their 
natatory bladders, because they cannot exhaust the 
* (.§ 57, note 2.] For the muscular system of the 
Acalephae, see also Forbes (loc. cit. p. 3), and 
Agassiz (loc. cit. p. 236). This last-named author 
has described this system with full details in many 
genera. It is much more complex than has hitherto 
been supposed, and I must refer for the details 
to the memoir in question. 
contained air. According to Olfers (Abhandl. d. 
Berl. Akad. 1831, p. 157, 165, Taf. I.), there are 
two of these bladders in Physalia, one of which 
only has an opening. Philippi (Miiller’s Arch. 
1843, p. 63) has found neither internal nor exter- 
nal opening to the bladder of Physophora tetras- 
ticha. In Stephanomia it would not appear, 
according to the description of Milne Edwards 
(Ann. d. Sc. Nat. XVI. p. 218, Pl. VIII. fig. 1. b. 
2), that this organ had an external opening. Couch 
(Froriep’s neue Notizen, No. 273, p. 129) denies 
that Physalia has the power to control the air of 
its bladder. See also below, § 65. 
2 Grant, Trans. Zool. Soc. London, ¥. 1835, p. 9.3 
Sars, Beskrivelser loc. cit. Pl. VIII. fig. 18, e.; 
Milne Edwards, Ann. d. Sc. Nat. XVI. p. 201, 
216, Pl. IV. fig. 2, 8, Pl. VI. fig. 1.¢.; and WZ, loc. 
cit. p. 9, 56, Taf. I. fig. 5. 
In regard to the structure of these muscles, 
Agassiz remarks : ‘“ With all the power of the best 
Oberhduser Microscope, I have been unable to dis- 
cover the slightest indication of striae on the nius- 
cular cells; nevertheless, it cannot be doubted 
that they are voluntary muscles.” To this view I 
may add my own of the same nature. — Ep. 
