62 



THE ACALEPHAE. 



^^ 57, 58. 



CHAPTER II. 



MUSCULAR SYSTEM AND ORGANS OF LOCOMOTION. 



§ 57. 



The Acalephae have a distinct muscuhxr 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. 



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 aerial 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.*-' 



1 Will (loc. cit. p. 48, Taf. I. fig. 11) has observed 

 in the contractile excrescences of the Eucharis, 

 not only circular fibres and numerous longitudinal 

 muscles, but large transversely-flattened ones, 

 which were bound together by oblique bands. 



^ Will, loc. cit. p. 47, 63, Taf. I. fig. 13, Accord- 

 ing to Wagner (Ueber den Bau, kc. ; 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 locomotion. The swimming is e.xclusively 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, littoral. 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 exliaust the 



contained air. According to Olfers (Abhandl. d. 

 Berl. Akad. 1831, p. 1.57, 165, Taf. I.), there are 

 two of these bladders in Physalia, one of which 

 only has an opening. Philippi {Muller's Arch. 

 1843, p. 63) has found neither internal nor exter- 

 nal opening to the bladder of Pliysophora tetrax- 

 ticha. In Stepkanomia it would not appear, 

 according to the description of Milne Edwards 

 (Ann. d. Sc. Nat. XVI. p. 218, PI. 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, 1. 1835, p. 9.; 

 Sars, Beskrivelser loc. cit. PI. VIII. fig. 18, e.; 

 Milne Edwards, Ann. d. Sc. Nat. XVI. p. 201, 

 216, PI. IV. fig. 2, 3, PI. VI. fig. 1. c; and Will, loo. 

 cit. p. 9, 56, Taf. I. tig. 5. 



* [ § 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. 



In regard to the structure of these muscles, 

 Agassiz remarks : " With all the power of the best 

 Oberhäuser Microscope, I have been unable to dis- 

 cover the slightest indication of striae on the mus- 

 cular cells ; nevertheless, it cannot be doubted 

 that they are voluntary muscles." To this view I 

 may add my own of the same nature. — Ed. 



