- The genus Monobrachium is a colonyforming hydroid which grows upon the shells of bivalve molluscs, each polyp having but a single tentacle.
In addition to insects, various kinds of worms, molluscs, &c., are sometimes of importance as pests.
Thus in the Chaetopoda the perivisceral cavity is coelomic; in this respect the group contrasts with the Arthropoda and Molluscs, where the perivisceral cavity is, mainly at least, part of the vascular or haemal system, and agrees with the Vertebrata.
The minute structure of the epithelium which clothes it, as well as the origin of the nerve which is distributed to the parabranchia, proves it to be the same organ which is found universally in molluscs at the base of each gill-plume, and tests the indrawn current of water by the sense of ?,g smell.
Our figure of the nervous system of Aplysia does not give the small pair of buccal ganglia which are, as in all glossophorous Molluscs, present upon the nerves passing from the cerebral region to the odontophore.
In Scotland the word "slug" is absent from the vernacular vocabulary, both shell-bearing and shell-less inland molluscs being known as snails.
The depths of the Black Sea are lifeless, higher organic life not being known to exist below loo fathoms. Fossiliferous remains of Dreissena, Cardium and other molluscs have, however, been dredged up, which help to show that conditions formerly existed in the Black Sea similar to those that exist at the present day in the Caspian Sea.
Molluscs are extraordinarily numerous; and many, both of water and land, are rarities among their kind for size and richness of colour.
Lime is, in fact, absorbed to an enormous extent by fishes, molluscs, crustacea, calcareous algae and sponges, starfishes, sea-urchins and feather stars, many polyzoa and a multitude of protozoa (mainly the foraminifera).
4 The Sea of Okhotsk is very interesting, owing to its local species and the general composition of its fauna (70 species of Molluscs and 21 of Gasteropods).
The land Molluscs; notwithstanding the unfavourable conditions of climate, number about seventy species - Siberia in this respect being not far behind north Europe.
This body occurs also in the blood of Crustacea and of Molluscs, but its abundance in both Limulus and Scorpio is very marked, and gives to the freshly-shed blood a strong indigo-blue tint.
Many Acari are parasitic on marine and freshwater molluscs, and others are found on the feathers of birds and the hair of mammals.
Cuvier dissected many Molluscs and other animals which had not previously been anatomized; of others he gave more correct accounts than had been given by earlier writers.
He showed (1830) that the organisms like Flustra are not hydroid Polyps, but of a more complex structure resembling Molluscs, and he gave them the name Polyzoa.
Molluscs are common on the coasts, including the pearl oyster, and in the fresh-water streams and lakes.
(A and B from Lankester's Treatise on Zoology, part iv., C original.) of proglottides or of eggs which are disseminated along with the faeces of the final host and subsequently eaten by herbivorous or omnivorous mammals, insects, worms, molluscs or fish.
The Elasmobranchs swallow infected molluscs or fish; pike and trout devour smaller fry; birds pick up sticklebacks, insects and worms which contain Cestode larvae; and man lays himself open to infection by eating the uncooked or partially prepared flesh of many animals.
Land and marine molluscs are numerous, and include various edible kinds.
The larvae usually live in Molluscs, the mature worm in vertebrates, and the immature but metamorphosed Trematode in either host and also in pelagic and littoral marine and fresh-water invertebrates.
As for the land and fresh-water molluscs, some 200 of which are known, they are mainly kindred with those of China and Siberia, tropical and Indian forms being exceptional.
The mussel (i-no.kai) is well represented by the species numa-gai (marsh-mussel), karasu-gai (raven-mussel), kamisori-gai (razor-mussel), shijimi-no-kai (Corbicula), of which there are nine species, &c. Unlike the land-molluscs, the great majority of Japanese sea-molluscs are akin to those of the Indian Ocean and the Malay archipelago.
Pure calcareous sand and calcareous mud are formed by wave action on the shores of coral islands where the only material available is coral and the accompanying calcareous algae, crustacea, molluscs and other organisms secreting carbonate of lime.
The land molluscs show relationship with the Indian and the Malayan sub-regions; but many forms have here their centre, and have spread hence into Australia and the Pacific islands.
Birds and mammals take the first place; the leading collections devote a good deal of attention to reptiles and batrachians; fishes and aquatic invertebrata are most often to be found only when there are special aquaria, whilst non-aquatic invertebrates are seldom to be seen and at most consist of a few moths and butterflies, spiders, scorpions and centipedes, molluscs and crustaceans.
Recognition of this threefold character led Deshayes to establish a threefold division of the Tertiary based on the percentage of molluscs belonging to types now living found in each.
De Blainville added to the knowledge of the shells of the Paris basin; Giovanni Battista Brocchi (1772-1826) in 1814, and Luigi Bellardi (1818-1889) and Giovanni Michelotti (born 1812) in 1840, described the Pliocene molluscs of the subApennine formation of Italy; from Germany and Austria appeared the epoch-making works of Heinrich Ernst Beyrich (1815-1896) and of Moritz Hoernes (1815-1868).
He recognized the fact that the shells of molluscs, which grow by successive additions, preserve unchanged the whole series of stages of their individual development, so that each shell of a Cretaceous ammonite, for example, represents five stages of progressive modification as follows: the first is the periode embryonnaire, during which the shell is smooth; the second and third represent periods of elaboration and ornamentation; the fourth is a period of initial degeneration; the fifth and last a period of degeneration when ornamentation becomes obsolete and the exterior smooth again, as in the young.
Until comparatively recent times the molluscs were considered as appearing on the limits of the Cambrian and Ordovician; but Charles D.
Cephalopod molluscs have been traced back to the straight-shelled nautiloids of the genus Volborthella, while true ammonites have been found in the inferior Permian of the Continent and by American palaeontologists in the true coal measures.
The minute gradations observed by Hyatt, Waagen and all invertebrate palaeontologists, in the hard parts (shells) of molluscs, &c., are analogous to the equally minute gradations observed by vertebrate palaeontologists in the hard parts of reptiles and mammals.
Un like the molluscs and annelids, however, the cloacal invagination lies outside this region, and the foot is formed by an elongation of the end of the body between the two apertures.
The departure of the Ordovician life from that of the Cambrian was perhaps most pronounced in the great development of the molluscs and crinoids (including cystoids), but corals were also abundant for the first time, and graptolites came into prominence.
Corals, echinoderms, brachiopods and all groups of molluscs abounded.
Among the more important features of the marine life of the period were (1) the great development of the molluscs, especially of cephalopods; (2) theabundanceoflargebrachiopods; (3) theaberrant tendencies of the trilobites; (4) the profusion of corals; and (5) the abundance, size and peculiar forms of the fishes.
Molluscs are usually of separate sexes, but sexual dimorphism is seldom highly developed.
More than 28,000 species of living Molluscs have been distinguished, of which more than half are Gastropods.
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.
The largest pair of branchiae is placed immediately behind the renal openings and corresponds to the single pair of other molluscs, the organs being repeated anteriorly only (Metamacrobranchs) or anteriorly and posteriorly (Mesomacrobranchs).
As in other molluscs the coelom is represented by a large pericardial cavity, situated above the intestine posteriorly, and a generative sac which is single and median and situated in front of the pericardium, except in the Nuttalochiton hyadesi, where the gonads are in a similar position, but are paired.
As in other molluscs, the arteries do not extend far, but lead into inter-visceral blood-spaces.
Other marine forms are Rhizopoda (Rotalia and Textillaria), the sponge Amorphina, the Amphicteis worm, the molluscs Cardium edule and other Cardidae, and some Amphipods (Cumacea and Mysidae,), but they are forms which either tolerate variations in salinity or are especially characteristic of brackish waters.
These include Protozoa, three sponges, Vermes, twenty-five Molluscs, numerous Amphipods, fishes of the genera Gobius, Benthophilus and Cobitis, and one mammal (Phoca caspia).
Its food consists of frogs, fish, fresh-water molluscs and crustaceans, as well as mice, rats, muskrats, rabbits and small birds.
A few rats and bats represent the indigenous mammals, but the sea is rich in fish and molluscs; and Dr Otto Finsch (Journ.
By the aid of this the larva makes its way into the soft body of some insect larva, Ephemerids, Chironomids, or even of Molluscs, and encysts in the muscles or fat body.
The surrounding seas contain great numbers of fish; the coral reefs abound with a great variety of molluscs; and there are numerous land-shells.
Fresh-water and marine molluscs are also very numerous.
The land molluscs have been thoroughly classified, but much still remains to be done with the marine species.
Remains of molluscs still extant in the Caspian occur extensively throughout this depression and up the lower Kama.