How to use Lamellibranchs in a sentence
Protohydra occurs in oysterbanks and Monobrachium also grows on the shells of bivalves, and both these hydroids probably fish in the currents produced by the lamellibranchs.
This columella muscle is the same thing as the muscles adhering to the shell in Patella, and the posterior adductor of Lamellibranchs.
This organ is probably homologous with the byssogenous gland of Lamellibranchs.
The embryo is provided with ten hooks, and appears to select Lamellibranchs (Mactra) for its intermediate host.
These organisms live in cockles, oysters and other lamellibranchs and they so affect the gonads of these molluscs as to castrate and sterilize their host.Advertisement
A different but still more interesting result is produced by these Trematode larvae on certain lamellibranchs.
In many embryo Lamellibranchs a centro-dorsal primitive shell-gland or follicle has been detected.
No accessory glands or copulatory organs are ever present in Lamellibranchs.
In many Lamellibranchs a gland is found on the hinder surface of the foot in the mid line, which secretes a substance which sets into the form of threads - the so-called " byssus " - by means of which the animal can fix itself.
In some Lamellibranchs (Pecten, Spondylus, Pholas, Mactra, Tellina, Pectunculus, Galeomma, &c.), although cephalic eyes are generally absent, special eyes are developed on the free margin of the mantle-skirt, apparently by the modification of tentacles commonly found there.Advertisement
This is not always the case with Lamellibranchs; there is in the group a tendency for the corresponding edges of the mantle-skirt to fuse together by concrescence, and so to form a more or less completely closed bag, as in the Scaphopoda (Dentalium).
In such Lamellibranchs as the oysters, scallops and many others which have the edges of the mantleskirt quite free, there are numerous tentacles upon those edges.
The hinder adductor muscle is always large in Lamellibranchs, but the anterior adductor may be very small (Heteromya), or absent altogether (Monomya).
In fact all Lamellibranchs begin with a condition in which there is only one adductor, and that not the posterior but the anterior.
The single adductor muscle of the Monomya is separated by a difference of fibre into two portions, but neither of these can be regarded as possibly representing the anterior adductor of the other Lamellibranchs.Advertisement
The ligament is simple in Anadonta; in many Lamellibranchs it is separated into two layers, an outer and an inner (thicker,and denser).
Whilst the valves of the shell are equal in Anodonta we find in many Lamellibranchs (Ostraea, Chama, Corbula, &c.) one valve larger, and the other smaller and sometimes flat, whilst the larger shell may be fixed to rock or to stones (Ostraea, &c.).
Other Lamellibranchs may have a larger foot relatively than has Anodonta.
These organs are characteristic of all Lamellibranchs; they do not vary except in size, being sometimes drawn out to streamer-like dimensions.
There is no embryological evidence to support this suggested connexion, and, as will appear immediately, the history of the gill-plates in various forms of Lamellibranchs does not directly favour it.Advertisement
The gill-plates have a structure very different from that of the labial tentacles, and one which in Anodonta is singularly complicated as compared with the condition presented by these organs in some other Lamellibranchs, and with what must have been their original condition in the ancestors of the whole series of living Lamellibranchia.
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.
Such a subdivision of the pallial chamber, and direction of the currents set up within it do not exist in a number of Lamellibranchs which have the gill-lamellae comparatively free (Mytilus, Arca, Trigonia, &c.), and it is in these forms that FIG.17.
The coil of the intestine in Anodonta is similar to that of other Lamellibranchs.
This is not an unusual arrangement in Lamellibranchs, and a similar disposition occurs in some Gastropoda (Haliotis).Advertisement
The renal organs may be more ramified in other Lamellibranchs than they are in Anodonta.
In some Lamellibranchs the osphradial ganglia receive nerve-fibres, not from the visceral ganglia, but from the cerebral ganglia along the visceral commissure.
The otocysts of Cyclas are peculiarly favourable for study on account of the transparency of the small foot in which they lie, and may be taken as typical of those of Lamellibranchs generally.
This deficiency is very usual in the class; at the same time, many Lamellibranchs have tentacles on the edge of the mantle supplied by a pair of large well-developed nerves, which are given off from the cerebro-pleural ganglion-pair, A, When free swimming, shows the two dentigerous valves widely open.
In some Lamellibranchs - for instance, the European Oyster and the Pisidium pusillum - the sexes are united in the same individual; but here, as in most hermaphrodite animals, the two sexual elements are not ripe in the same individual at the same moment.
In the most primitive Lamellibranchs there is no separate generative aperture but the gonads discharge into the renal cavity, as in Patella among Gastropods.
This byssus is not homologous with that of other Lamellibranchs, but originates from a single glandular epithelial cell embedded in the tissues on the dorsal anterior side of the adductor muscle.
The early larva of Anodonta is not unlike the trochosphere of other Lamellibranchs, but the mouth is wanting.
The latter were regarded as forming a distinct branch, equivalent in rank to the other three classes together, the latter all possessing the radula which is wanting in Lamellibranchs.
It is possible therefore to regard the latter condition as secondary, and to conclude that the separate genital ducts have been derived from the original single pair of coelomic ducts, as in Lamellibranchs.
The foot is a muscular mass without cuticle or skeleton, excepting certain cuticular structures such as the byssus of Lamellibranchs and the operculum of Gastropods, which do not aid in locomotion.
Hermaphroditism is secondary, and occurs in one sub-class of Gastropoda, in some Lamellibranchs, and in one sub-order of Amphineura.
As a rule no parental care is exhibited, but incubation of the developing ova within some part of the parental body, or receptacles attached to the parent, occurs in some Lamellibranchs, some Gastropods, and in Argonauta among the Cephalopods.
As a rule Molluscs are free and more or less active, but many Lamellibranchs are sedentary, and a few of these and of Gastropods are permanently fixed to their habitat.
Just behind the ciliated ring is a pair of larval eyes which disappear in the adult; these correspond to the cephalic eyes of Lamellibranchs.
On this view then the Aplacophora are more primitive than the Polyplacophora in the relations of coelom, gonad and coelomoducts; and the genital ducts of the Chitons have arisen either by metameric repetition within the group, or by the gradual loss of an original connexion between the generative sac and the renal tube, as in Lamellibranchs and Gastropods, the generative sac acquiring a separate duct and opening to the exterior on each side.
The degeneration produced by sedentary habits in all lamellibranchs has in the oyster reached its most advanced stage.
In a corresponding manner the right side of the animal's body is somewhat less developed than the left, and to this extent there is a departure from the bilateral symmetry characteristic of Lamellibranchs.
In Anodon and the majority of lamellibranchs the ventricle surrounds the intestine; in the oyster the two are quite independent, the intestine passing above the pericardium.
The renal organs of the oyster were discovered by Hoek to agree in their morphological relations with those of other lamellibranchs.
The gonad, whose position has already been mentioned, is divided into transverse lobes; its duct is anterior and single, and diverges to the right to open into the right kidney as in primitive Gastropods and Lamellibranchs.
A good case for the examination of the question as to whether blood enters the pericardium of Lamellibranchs, or escapes from the foot, or by the renal organs when the animal suddenly contracts, is furnished by the Ceratisolen legumen, which has red blood-corpuscles.