Usually only one orifice to the lachrymal canal, situated inside the rim of the orbit.
There is but very little food-material in the egg of this Pectinibranch, and consequently the diblastula forms by invagination; the blastopore or orifice of invagination coincides with the anus, and never closes entirely.
Os, s, Orifice formed by the un f, Metapodium.
In the winter it covers the orifice of this burrow with a layer of silk, and lies dormant underground until the return of spring.
By means of this instrument the time occupied in the flow of a measured quantity of the oil through a small orifice at a given temperature is measured.
This process, which we have followed as it takes place on an individual portion of the falling liquid, goes through its several phases at different distances from the orifice, so that if we examine different portions of the stream as it descends, we shall find next the orifice the unbroken column, then a series of contractions and enlargements, then elongated drops, then flattened drops, and so on till the drops become spherical.
In its simplest form this is a hollow flask-shaped horny piece, consisting of a dilated basal portion and a terminal spiniform portion with an orifice at the apex; but its structure is frequently complicated by accessory processes and outgrowths which aid copulation and serve to protect the delicate point from injury.
In one experiment the jet issued horizontally from an orifice of about half a centimetre in diameter, and almost immediately assumed a rippled outline.
The organs are developed as invaginations of the epidermis of the foot, and in the majority of the Protobranchia the orifice of invagination remains open throughout life; this is also the case in Mytilus including the common mussel.
Short, truncate in front and behind; cloacal orifice transverse; gills present; rather thin cuticle; no radula.