The sub-class is now divided into two orders: the Aspidobranchia in which the branchia or ctenidium is bipectinate and attached only at its base, and the Pectinibranchia in which the ctenidium is monopectinate and attached to the mantle throughout its length.
17, d), we find right and left of the two renal apertures a right and left gillplume or ctenidium, which here as in Haliotis and Pleurotornaria retain their original paired condition.
It will be remembered that, according to Spengel, the osphradium of mollusca is definitely and intimately related to the gill-plume or ctenidium, being always placed near the base of that organ; further, Spengel has shown that the nerve-supply of this olfactory organ is always derived from the visceral loop. Accord ingly, the nerve-supply FIG.
- A, Section in a plane vertical to the surface of the neck of Patella through a, the rudimentary ctenidium (Lankester's organ), and b, the olfactory epithelium (osphradium); c, the olfactory (osphradial) ganglion.
(After Spengel.) B, Surface view of a rudimentary ctenidium of Patella excised and viewed as a transparent object.
A single bipectinate ctenidium on left side.
Shell spirally coiled; a single ctenidium; eyes perforated; a horny operculum; lobes between the tentacles.
Shell with very low spire, without umbilicus, internal partitions frequently absorbed; a single ctenidium; a cephalic penis present.
No ctenidium, but a pulmonary cavity; heart with a single auricle, not traversed by the rectum.
No ctenidium, but a pulmonary cavity; operculum with an npophysis.
Br, Ctenidium (gill-plume).
This organ has, without reason, been supposed to represent the second ctenidium of the typical mollusc, which it cannot do on account of its position.
The Pectinibranchia the pedal nerves are br, Ctenidium (branchial distinctly nerves given off from the pedal plume).
U, The otocyst attached to the Ctenidium (gill-plume).
No ctenidium, pallial cavity transformed into a lung; aperture of shell circular; terrestrial.
To the left of the ctenidium a pulmonary sac, separated from it by an incomplete septum, am phibious.
Mantle with two posterior appendages; ctenidium large and capable of protrusion from pallial cavity.
Shell turriculated, with carinated br, Ctenidium (branchial whorls, the carinae tuberculated or plume).
Ctenidium of typical form Pleurocera.
I, Ctenidium (gill-plume).
34.-Female Janthina, with egg-float (a) attached to the foot; b, egg-capsules; c, ctenidium (gill-plume); d, cephalic tentacles.
Where the modification is carried to its extreme degree, not only the shell but the pallial cavity, ctenidium and visceral hump disappear, and the body acquires a simple elongated form and a secondary external symmetry, as in Pterotrachaea and in Doris, Eolis, and other Nudibranchia.
(All from Lankester.) by the mantle-skirt, is the ctenidium with its free end turned backwards.
The heart lies in front of, instead of to the side of, the attachment of the ctenidium - hence Opisthobranchia as opposed to " Prosobranchia," which correspond to the Streptoneura.
Many Opisthobranchia have by a process of atrophy lost the typical ctenidium and the mantleskirt, and have developed other organs in their place.
The gill-plume,which in A plysia is the typicalMolluscan ctenidium, is seen in fig.
The ctenidium (branchial plume).
There is a ctenidium, except in some Thecosomata and Gymnosomata, and an osphradium.
The pallial cavity is always well developed, and contains the ctenidium, at least in part; ctenidium, except in Lophocercidae, of folded type.
Cephalic shield continuous with dorsal integument; no shell; ctenidium projecting from mantle cavity.
Peraclis, ctenidium present.
Foot without parapodia; no pallial cavity, but always a single ctenidium situated on the right side between mantle and foot.
Shell external and conical; anterior tentacles form a frontal veil; ctenidium extending only over right side; a distinct osphradium.
Shell external, conical, much flattened; anterior tentacles very small, and situated with the mouth in a notch of the foot below the head; ctenidium very large.
Shell absent in the adult; no ctenidium or osphradium.
The ctenidium is atrophied, and the edge of the mantle-skirt is fused to the dorsal integument by concrescence, except at one point which forms the aperture of the mantle-chamber, thus converted into a nearly closed sac. Air is admitted to this sac for respiratory and hydrostatic purposes, and it thus becomes a lung.
Pulmonata are widely distinguished from a small number of Streptoneura at one time associated with them on account of their mantle-chamber being converted, as in Pulmonata, into a lung, and the ctenidium or branchial plume aborted.
The position of the osphradium corresponds more or less closely with that of the vanished right ctenidium, with which it is normally associated.
The Molluscan ctenidium is typically a plume like structure, consisting of a vascular axis, on each side of which is set a row of numerous lamelliform or filamentous processes.
Arca and Pectunculus) the lateral processes which are set on the axis of the ctenidium are not lamellae, but are slightly flattened, very long tubes or hollow filaments.
II, A a portion of four filaments of a ctenidium of the sea-mussel (Mytilus) is represented, having precisely the same structure as those of Arca.
The filaments of the gill (ctenidium) of Mytilus and Arca thus form two closely set rows which depend from the axis of the gill like two parallel plates.
As the axis of the ctenidium lies by the side of the body, and is very frequently connate with the body, as so often happens in Gastropods also, we find it convenient to speak of the two plate-like structures formed on each ctenidial axis as the outer and the inner gill-plate; each of these is composed of two lamellae, an outer (the reflected) and an adaxial in the case of the outer gill plate, and an adaxial and an inner (the reflected) in the case of the inner gill-plate.
Section across the axis of a ctenidium with a pair of plates - flattened and shortened filaments - attached.
I,j, k,g Are placed on or near the membrane which attaches the axis of the ctenidium to the side of the body.
Although the structured of the ctenidium is thus highly complicated in Anodonta, it is yet more so in some of the siphonate genera of Lamellibranchs.
Other points in the modification of the typical ctenidium must be noted in order to understand the ctenidium of Anodonta.
The axis of each ctenidium, right and left, starts from a point well forward FIG.
- Filaments of the Ctenidium of Mytilus edulis.
If we were to make a vertical section across the long axis of a Lamellibranch which had the axis of its ctenidium free from its origin onwards, we should find such relations as are shown in the diagram fig.