In zoology, the mollusca are divided into cephalous and acephalous (Acephala), according as they have or have not an organized part of their anatomy as the seat of the brain and special senses.
Turtle and sea-snakes abound, as do mollusca, of which a few are peculiar, and zoophytes.
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.
Time when the fact that the renal organ s,s', Nerves (right and of the Mollusca, as a rule, opens into the left) to the mantle.
The general structure of the Molluscan intestine has not been sufficiently investigated to render any comparison of this structure of Patella with that of other Mollusca possible.
The histology of the nervous system of Mollusca has yet to be seriously inquired into.
In other Pectinibranchia (and such variations are representative for all Mollusca, and not characteristic only of Pectinibranchia) we find that there is a very unequal division of the egg-cell at the commencement of embryonic development, as in Nassa.
The general features of this process and of the relation of the blastopore to mouth and anus have been explained in treating of the development of Mollusca generally.
This is the largest group of Mollusca, including nearly sixty families, some of which are insufficiently known from the anatomical point of view.
The Pteropoda, formerly regarded as a distinct class of the Mollusca, were interpreted by E.
In deed, were it not for their retention of the characteristic odontophore we should have little or no indication that such forms as Phyllirhoe and Limapontia really belong to the Mollusca at all.
In one genus (Planorbis) the plasma of the blood is coloured red by haemoglobin, this being the only instance of the presence of this body in the blood of Glossophorous Mollusca, though it occurs in corpuscles in the blood of the bivalves Arca and Solen (Lankester).
60), at one time supposed to be the developing anus, but shown by Lankester to be identical with the " shell-gland " discovered by him in other Mollusca (Pisidium, Pleurobranchidium, Neritina, &c.).
Whether the closed primitive shell-sac of the slugs (and with it the transient embryonic shell-gland of all other Mollusca) is precisely the same thing as the closed sac in which the calcareous pen or shell of the Cephalopod Sepia and its allies is formed, is a further question which we shall consider when dealing with the Cephalopoda.
In most other Mollusca (Anisopleurous Gastropods, Pteropods and Conchifera) there is a want of such continuity; the primitive shell-sac contributes no factor to the permanent shell, or only a very minute FIG.
(After Spengel.) being formed afresh on the surface of the visceral hump. It is, then, in this sense that we may speak of primary, secondary and tertiary shells in Mollusca, recognizing the fact that they may be merely phases fused by continuity of growth so as to form but one shell, or that in other cases they may be presented to us as separate individual things, in virtue of the non-development of the later phases, or in virtue of sudden changes in the activity of the mantle-surface causing the shedding FIG.
Herdman, " On the Structure and Functions of the Cerata or Dorsal Papillae in some Nudibranchiate Mollusca," Quart.
P. Woodward, Manual of the Mollusca (2nd ed., with appendix, London, 1869); E.
Ray Lankester, " Mollusca," in 9th ed.
- Diagrams of the five classes of Mollusca, from the left side.
For speciegraphical details, see Woodward's Manual of the Mollusca (1875), and Bronn's Tierreich (Weichtiere).
Numerous raised beaches and terraces, containing shells of marine mollusca, &c., occur along the whole coast of Greenland, and indicate that the whole of this large island has been raised, or the sea has sunk, in post-glacial times, after the inland ice covered its now icebare outskirts.
" Conchiolin," the matrix of shells of the mollusca, is only slightly soluble in acids.
Brachiopods have been at various times placed with the Mollusca, the Chaetopoda, the Chaetognatha, the Phoronidea, the Polyzoa, the Hemichordata, and the Urochordata.
Associated with Ray in his work, and more especially occupied with the study of the Worms and Mollusca, was Martin Lister (1638-1712), celebrated also as the author of the first geological map.
He upset (1830) Cuvier's retention of the Cirripedes among Mollusca, and his subsequent treatment of them as an isolated class, by showing that they begin life as free-swimming Crustacea identical with the young forms of other Crustacea.
These Trematodes occur in the alimentary canal and adjacent organs of Mollusca, the gall-bladder of Chimaera, and the intestine of Chelonia and of certain fish.
This appears to indicate B that the Polyzoa are remotely allied to other phyla in which this type of larva prevails, and in particular to the Mollusca and Chaetopoda, as well as to the Rotifera, which are regarded as persistent Trochospheres.
These, like the mollusca, indicate the influence of the Kuro Shiwo and the south-west monsoon, for they have close affinity with species found in the Indian and Pacific Oceans.
Malta has several species of zoophytes, sponges, mollusca and crustacea.
De Pourtales pointed out in 1870; they consist of remains of deep-sea corals, serpulae, echinoderms and mollusca united Sands, gravels, muds, &c.
Murray and Renard define globigerina ooze as containing at least 30% of calcium carbonate, in which the remains of pelagic (not benthonic) foraminifera predominate and in which remains of pelagic mollusca such as pteropods and heteropods, ostracodes and also coccoliths (minute calcareous algae) may also occur.
He was also author (with John Lycett) of A Monograph of the Mollusca from the Great Oolite (Palaeontographical Soc., 1850-1853).
In the animal kingdom it occurs as both calcite and aragonite in the tests of the foraminifera, echinoderms, brachiopoda, and mollusca; also in the skeletons of sponges and corals.
In the latter respect, and in the fact that they frequently develop by a metamorphosis, they approach the Mollusca, but they differ from that group notably in the occurrence of metameric segmentation affecting many of the systems of organs.
From a study of remains of the mollusca, brachiopoda and other marine organisms they will determine the shallow water (littoral) and deep water (abyssal) regions of the surrounding oceans, and the clear or muddy, salt, brackish or fresh character of its inland and marginal seas; and even the physical conditions of the open sea at the time will be ascertained.
It, as in other Mollusca, is not a blood-space but develops from the coelom, and it communicates with the exterior by the pair of renal tubes.
As in Cephalopoda (and possibly other Mollusca) water can be introduced through the nephridia into this space.
They are totally distinct from the cephalic eyes of typical Mollusca, and have a different structure and historical development.
21), so that its fibres join the anterior faces of the nerve-end cells as in Vertebrates, instead of their posterior faces as in the cephalic eyes of Mollusca and Arthropoda; moreover, the lens is not a cuticular product but a cellular structure, which, again, is a feature of agreement with the Vertebrate FIG.
Peck, " The Minute Structure of the Gills of Lamellibranch Mollusca," Quart.
Iii.; Paul Pelseneer, " Mollusca," Treatise on Zoology, edited by E.
MOLLUSCA, one of the great " phyla," or sub-kingdoms, of the animal pedigree or kingdom.
The shell-bearing forms belonging to this group which were known to Linnaeus were placed by him (in 1748) in the third order of his class Vermes under the name " Testacea," whilst the Echinoderms, Hydroids and Annelids, with the naked Mollusca, formed his second order termed " Zoophyta."
Ten years later he replaced the name " Zoophyta " by " Mollusca," which was thus in the first instance applied, not to the Mollusca at present so termed, but to a group consisting chiefly of other organisms. Gradually, however, the term Mollusca became used to include those Mollusca formerly placed among the " Testacea," as well as the naked Mollusca.
It is important to observe that the term µaXaiaa, of which Mollusca is merely a latinized form, was used by Aristotle to indicate a group consisting of the cuttle-fishes only.
As now classified, the Mollusca consist of the following subdivisions: - Grade A.
The definite erection of the Mollusca into the position of one of the great primary groups of the animal kingdom is due to George Cuvier (1788-1800), who largely occupied himself with the dissection of representatives of this type.