MOLLUSCA , one of the See also:

• GREAT

See also:

• HISTORY

The name of J. Vaughan Thompson is again that which is primarily connected with the history of a Molluscan class. In 183o he pointed out that among the numerous kinds of "polyps" at that time associated by naturalists with the Hydroids, there were many which had a See also:

• PECULIAR

In See also:

• ENGLAND
• ENGLAND, THE CHURCH OF

Brit. was published important advances have been made in our knowledge of the Mollusca, as the result of researches largely due to the See also:

• INTEREST

In the Coelentera, whatever subsequent changes of shape the little sac may undergo as it grows up to be See also:

• POLYP

The coelom opens to the exterior by ducts which are primarily genital ducts by which the ova or sperms are discharged. These ducts, however, as well as the coelomic epithelium, may assume excretory functions. In Mollusca the coelom is reduced and consists of two Tarts, the pericardial cavity which surrounds the See also:

• HEART
• HEART, LUNG

Among the Gastropoda, in the Aspidobranchia, there is no genital duct, and the gonad opens into the right See also:

• KIDNEY

The Dibranchia, with only one pair of branchiae, one pair of renal organs, and one pair of genital ducts, are much more See also:

• RECENT

The See also:

• REST (O. Eng. rest, reste, bed, cognate with other Teutonic forms, e.g. Ger. Rast, Riiste, rest, and probably Gothic Rasta, league, i.e. resting or stopping place)

The external epithelium of the dorsal region secretes the shell. Between the edge of the shell and the foot there is a groove or cavity, chiefly See also:

• DEVELOPED

In one or two cases the epithelium of the foot secretes a calcified shell, which is either free as in Argonaula or adherent as in Hipponyx. The ctenidia (fig. r) are the branchial organs of the Mollusca. In the primitive condition there is one on each side in the mantle cavity, towards the posterior end of the body. Each is an outgrowth of the body-wall at the side of the body, and consists of an axis containing two main vessels, an afferent and efferent, and bearing on either side a series of transverse plates whose blood-sinuses communicate with the vessels of the axis. The afferent vessel of the ctenidium receives blood from the vena cava or See also:

• PRINCIPAL

It was present in the ancestral mollusc, occurs in nearly all archaic types, and is only absent in the most specialized forms, in which it has evidently been lost; these forms are certain Neomeniomorpha, all the Lamellibranchia, various degenerate Gastropoda, and the Cirrhoteuthidae among Cephalopods. The teeth are secreted by a small number of cells at the closed end of the caecum, the basal membrane by a trans-See also:

• VERSE (from Lat. versus, literally a line or furrow drawn by turning the plough, from vertere, and afterwards signifying an arrangement of syllables into feet)

It consists of a median ventricle with muscular walls and a cavity traversed by muscular strands. On either side of the ventricle, in the primitive condition, is a thin-walled auricle, opening into the ventricle by a valved opening. Each auricle forms the terminal enlargement of the efferent vein of the ctenidium of its own side. In Nautilus two pairs of auricles are present, corresponding with the two pairs of ctenidia. In the primitive form a single anterior aorta is given off from the ventricle, the two together representing the dorsal blood-vessel of Chaetopods. In more specialized forms a posterior aorta passes backwards from the ventricle, as in Gastropods and the majority of Lamellibranchs. The ramifications of the arteries convey the blood to all parts of the body, and it finally reaches the venous sinuses, the chief of which are the pedal, the pallial and the median-ventral. The last is between the pericardium and the foot; from it the blood passes through the renal organs to the ctenidia. Some blood, however, enters the auricles directly from the mantle, without passing through the ctenidia. In the majority of Gastropoda one gill and one auricle are lost. The blood is usually a colourless liquid containing amoeboid cells and sometimes other corpuscles called haematids. It may be coloured See also:

• BLUE (common in different forms to most European languages)

In a few forms the blood contains haemoglobin, either in See also:

• SOLUTION (from Lat. solvere, to loosen, dissolve)

There are usually three small ganglia on the course of this visceral dommissure, namely, the right and left visceral ganglia and the abdominal. The perioesophageal nerve-See also:

• RING (O.E. hring; a word common to Teutonic languages; and probably cognate with the Lat. circus, Gr. KtpKOS or KpLKOS, Skt'. chakra, wheel, circle, cf. also " harangue "); in art, a band of circular shape of varying sizes, made of any material and used f

Those cases which appear to be most typical —i.e. which adhere to a See also:

• PROCEDURE (Fr. procedure, from Lat. procedere, to go for-ward)

4. As the micromeres become more numerous they gradually envelop the megameres until the latter are completely enclosed. The gastrula is in these cases said to be formed by epibole. Between ectoderm and endoderm a third intermediate cell-layer A~~t_ ,o . /2 I See also:

• POND
• POND, JOHN (c. 1767-1836)

The mesoderm arises for the most part from the endoderm. When the segmentation is unequal one of the megameres gives rise by successive divisions to two primary mesoderm cells called mesomeres; these See also:

• DIVIDE