"$194. 



THE ACEPHALA. 



211 



These four brancKial lamellae, whose lower border is free while the other 

 is attached to the viscera, always extend along the abdomen, and not un-* 

 frequently come together above. <^^' 



Each lamella is formed, essentially, by a widely-projecting cutaneous 

 fold, the two leaves of which are connected by numerous transverse septa, to 

 which correspond externally as many furrows which pass from the base of 

 the branchia to its borders. All these furrows have upon each margin a 

 row of long cilia, which, upon the borders of the branchia, connect with an 

 ordinary ciliated epithelium. ^^^^ The compartments formed by these inter- 

 leaved septa are also lined with a very delicate ciliated epithelium, and 

 connect with the cavity of the mantle at the base of the branchiae.^i'^^ 



With an entire group of this order, the branchial structure is quite dif- 

 ferent from that just described. Externally, these organs appear like ordi- 

 nary branchiae, but examined more closely it will be found, that, instead of 

 lamellae, they are composed of numerous thickly-set ribands arranged in 

 rows.''*' These ribands are formed of two lamellae blended together at 

 their extremity. Their circumscribing space is without doubt solely for the 

 lodging of the branchial vessels ; for, at their base, there is no orifice anal- 

 ogous to those found in the other species of this order. The blood collects 

 at the base of the branchiae in the longitudinal canal, from which the lat- 

 eral vessels are given off, at right angles. Thence it passes into the branch- 

 iae, traversing a trellis-like net-work quite resembling the analogous one of 

 the Ascidiae.'^^* Another series of lateral vessels serves as the branchial 

 veins, pouring the blood into other longitudinal canals, whence it passes 

 into the two auricles of the heart. ^-"^ 



the different orifices of the mantle may be clearly 

 seen by observing these animals, when they, at rest, 

 protrude between the valves either their siphon or 

 the borders of the mantle and tinge the surround- 

 ing water with coloring matter, which makes the 

 •currents quite distinct. 



15 With Unio, Anodonta, Mactra, Cardium, 

 Isocardia, Lutraria, &c., the four branchial layers 

 are united at their posterior extremity. But with 

 Pecten, Avicula, Area, Pectuncutus, and Pin- 

 na, they are disconnected and extended backwards 

 by two free prolongations. 



16 These ciliary movements tend to carry the wa- 

 ter, with the internal branchiae, towards their free 

 border ; and with the external, towards their base. 



17 These orifices of the branchial compartments 

 are easily seen at the base of these organs ; ex- 

 cepting, however, those belonging to the two exter- 

 nal branchiae.which are concealed beneath a kind of 

 canal formed by the mantle. These two canals, 

 closed in front, open behind between the end of the 

 abdomen and the anus into that portion of the cavity 

 of the mantle leading lo the anal fissure or tube, and 

 •which maybe regarded as a Cloaca ; see Unio, 

 Anodonta, Fenus, Cardium, Isocardia, Mactra, 

 &.C. With many, as for instance with Unio, and 

 Anodonta, the compartments of the external bran- 

 chiae are much more developed than those of the 

 internal, and their orifices can be closed in a lip- 

 like manner by the vesicular enlargement of the 

 septa which limit them on each side. With Pinna, 

 the branchiae are quite different. Their leaves are 

 united by short filaments instead of by septa ; and 

 thus, although not divided into compartments, 

 there are orifices at their base which lead into the 

 interior, and which are situated upon the internal 

 surface of the interna!, and upon the external sur- 

 face of the external branchiae. 



18 This pectinated form of the branchiae has 

 been observed by Baer {MeckeCs Arch. 1830, p. 

 340), with Mytilus, and by Meckel (Syst. d. ver- 

 gleich. Anat. VI. p. 60), with Spondylus, Pecten, 

 and Area ; see, also, the Rögne animal de Cuvier, 

 nouv. Mit. Mollus(iues. PI. LXXIV. fig. 2, a. I 

 have seen similar branchiae with Pectunculus, 

 Avicula, and Lithodomus. Philippi (fVie^- 

 mann^s Arch. 1835, I. p. 274) has seen them even 

 still more developed with Solenomya. 



This peculiar branchial apparatus with Mytilus 

 has been described with much detail by Sharpey 

 (Cyclop, of Anat. I. p. 621). I have myself, dur- 

 ing the autumn of 1847, completely verified the 

 statements of this author, upon living specimens of 

 the genera, Mytilus, Area, and Pecten, and would 

 insist here only on a single remarkable fact. Each 

 riband-like branchial filament has, upon both of 

 its surfaces, several cap-like papillae by means of 

 which these filaments are united together in a trel- 

 lis-like manner. When the branchiae are forcibly 

 distended, the papillae of the filaments are separ- 

 ated from each other. But this separation is not 

 very extended, for there is a cord Cümposed of del- 

 icate fibres, between each two papillae and binding 

 them together ; but sometimes, from undue force, 

 this cord is broken in its middle, and then each 

 broken extremity appears as a bundle of moving 

 vibratile ciUa which projects from the cavity of the 

 capsule (see Sharpey, loc. cit. fig. 305, E. a). The 

 function of this apparatus, which ceases to be visi- 

 ble when the papillae are united together, is yet 

 wholly doubtful. 



I'J Unio, Anodonta, Lima, Pinna, Ostrea, &c. ; 

 see Treviranus, Beobacht. ausd. Zoot. &c. fig. 62, 

 63, 65 (Ostrea and Anodonta) ; and Pali, loc. cit. 

 Tab. IX. fig. 17 (Unio). 



20 For the branchial vessels, see, especially, Bo- 



