F, Gill lamellae (not ctenidia, but organs of the pallial complex, having two kidneys, in some cases two branchiae, and two auricles.
Spengel has, however, in a most ingenious way shown that these bodies are the representatives of the typical pair of ctenidia, here reduced to a mere rudiment.
This circlet of gill-lamellae led Cuvier to class the limpets as Cyclobranchiata, and, by erroneous identification of them with the series of metamerically repeated ctenidia of Chiton, to associate the latter mollusc with the former.
In these, as in Patella, the typical ctenidia are aborted, and the branchial function is assumed by close-set lamelliform processes arranged in a series beneath the mantle-skirt on either side of the foot.
Limpet, and that of the g ' nerves which pass from the visceral loop of Haliotis to the olfactory patch or osphradium, which lies in immediate relation on the right and on the left side to the right and left gill-plumes (ctenidia) respectively.
Thus, then, we find that the limpet possesses a symmetrically disposed pair of ctenidia in a rudimentary condition, and justifies its position among Aspidobranchia.
No ctenidia but pallial branchiae in a circle between mantle and foot.
Neither ctenidia nor pallial branchiae.
RHIPIDOGL0ssA.-Aspidobranchia with a palliovisceral anastomosis (dialyneurous); eye-vesicle closed, with crystalline lens; ctenidia, osphradia and hypobranchial glands paired or single.
Shell spiral; mantle and shell with an anterior fissure; two ctenidia; a horny operculum.
Spire of shell much reduced; two bipectinate ctenidia, the right being the smaller; no operculum.
Shell conical; slit or hole in anterior part of mantle; two symmetrical ctenidia; no operculum.
It corresponds to the right of the two primitive ctenidia in the untwisted archaic condition of the molluscan body, and does not project freely into the branchial cavity, but its axis is attached (by concrescence) to the mantle-skirt (roof of the branchial chamber).
The paired ctenidia are very greatly developed right and left of the elongated body, and form the most prominent organ of the group. Their function is chiefly not respiratory but nutritive, since it is by the currents produced by their ciliated surface that food-particles are brought to the feebly-developed mouth and buccal cavity.
The chief points in which they vary are - (1) in the structure of the ctenidia or branchial plates; (2) in the presence of one or of two chief muscles, the fibres of which run across the animal's body from one valve of the shell to the other (adductors); (3) in the greater or less elaboration of the posterior portion of the mantle-skirt so as to form a pair of tubes, by one of which water is introduced into the sub-pallial chamber, whilst by the other it is expelled; (4) in the perfect or deficient symmetry of the two valves of the shell and the connected soft parts, as compared with one another; (5) in the development of the foot as a disk-like crawling organ (Arca, Nucula, Pectunculus, Trigonia, Lepton, Galeomma), as a simple plough-like or tongueshaped organ (Unionidae, &c.), as a re-curved saltatory organ (Cardium, &c.), as a long burrowing cylinder (Solenidae, &c.), or its partial (Mytilacea) or even complete abortion (Ostraeacea).
A dorsal contractile heart, with symmetrical right and left auricles receiving aerated blood from the ctenidia and mantle skirt, is present, being unequally developed only in those few forms which are inequivalve.
Fissure between the free edge water forms which carry the young in brood-pouches formed by the ctenidia have suppressed this larval phase.
- Structure of the Ctenidia of Nucula.
- Diagram of a view from the left side of the animal of Anodonta cygnaea, from which the mantle-skirt, the labial tentacles and the gill-filaments have been entirely removed so as to show the relations of the axis of the gill-plumes or ctenidia g, h.
Probably a straining of water from solid particles is effected by the lattice-work of the ctenidia or gill-plates.
The four orders now retained exhibit successive stages in the modification of the ctenidia by reflection and concrescence of the filament, but other organs, such as the heart, adductors, renal organs, may not show corresponding stages.
The characteristic organs of Mollusca are the mantle and shell, the foot, the ctenidia and the radula, of which all but the last are external.
- Ctenidia of various Mollusca (original).
Buccal mass and radular apparatus are present, but ctenidia are entirely wanting.
P, foot; o, mouth (at the other end of the foot is seen the anus raised on a papilla); kr, oral fringe; br, the numerous ctenidia (branchial plumes); spreading beyond mentum which is porcellaneous, quite compact, and entirely covered by the tegmentum.
Br, Ctenidia (branchial plumes).
The body chamber (reduced pallial chamber, conis worm-like and cylindritaining the concealed_pair of ctenidia) cal, the posterior half a to the right.
The branchial current is maintained by the cilia which cover the surface of the ctenidia, except in Cephalopoda, in which cilia are absent and the current is due to muscular action.
The ctenidia, it will be observed, have not yet been mentioned, and they are indeed the last of the characteristic Molluscan organs to make their appearance.
The metameric repetition of the shellplates and of the ctenidia are probably special modifications, but it is difficult to explain the spicules of the dorsal integument except as a condition more primitive than the shell itself.