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.
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.
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.
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.
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.
The axis of the ctenidium is seen to be adherent to, or fused by concrescence with, the body-wall, and moreover on each side the outer lamella of the outer gill-plate is fused to the mantle, whilst the inner lamella of the inner gill-plate is fused to the foot.
It is important, because such a concrescence is by no means universal, and does not occur, for example, in Mytilus or in Arca; further, because when its occurrence is once appreciated, the reduction of the gill-plates of Anodonta to the plume-type of the simplest ctenidium presents no difficulty; and, lastly, it has importance in reference to its physiological significance.
Portion of the axis of the left ctenidium which is fused with the base of the foot, the two dotted lines indicating the origins of the two rows of gill-filaments.
The afferent vessel of the' ctenidium receives blood from the vena cava or principal blood-sinus of the body, the efferent vessel opens into the auricle of its own side.
Near the base of the ctenidium is a patch of sensory epithelium innervated from the branchial nerve, forming a sense-organ called the osphradium, whose function is to test the water entering the branchial cavity.
It consists of the anus in the middle, a renal organ and renal aperture on each side of this, and a ctenidium outside or anterior to the renal organ, an osphradium being situated at the base of the ctenidium.
Each auricle forms the terminal enlargement of the efferent vein of the ctenidium of its own side.
Each gill has the structure of a typical molluscan ctenidium, consisting of an axis bearing an anterior and posterior row of filaments or lamellae.
The single pair of branchiae are placed symmetrically right and left of the anus, and each has the structure of a ctenidium bearing a row of lamellae on each side as in the Polyplacophora.