Many points in the development and mechanism of the nematocyst are disputed, but it is tolerably certain (I) that the cnidocil is of sensory nature, and that stimulation, by contact with prey or in other ways, causes a reflex discharge of the nematocyst; (2) that the discharge is an explosive change whereby the in-turned thread is suddenly everted and turned inside out, being thus shot through the opening in the outer wall of the capsule, and forced violently into the tissues of the prey, or, it may be, of an enemy; (3) that the thread inflicts not merely a mechanical wound, but instils an irritant poison, numbing and paralysing in its action.
Fixation takes place by means of this sucker, which is everted for the purpose, part of its epithelium becoming the basal ectoderm of the ancestrula.
A, Siphonal notch of the shell e, Everted buccal introvert (prooccupied by the siphonal boscis).
Further, the Gastropod's introvert is pleurembolic (and therefore acrecbolic), and is limited both in eversion and in introversion; it cannot be completely everted owing to the muscular bands (fig.
The colour is generally black-and-tan or brownish, the body is extremely long and cylindrical; the ears are large and pendulous, the legs broad, thick and twisted, with everted paws..
In Nemertines the everted proboscis is retracted in the same way as the tip of a glove finger would be if it were pulled backwards by a thread situated in the axis and attached to the tip. The comparison may be carried still further.
Where the whole crop is strengthened by a framework of bars, the incudate mastax lies in a little postero-ventral pouch which can be everted through the crop and mouth.
Starfishes devour large numbers; they are able to pull the valves of the shell apart and then to digest the body of the oyster by their everted stomach.
At the metamorphosis, this sac is everted and the alimentary canal is drawn into it in the form of a loop (fig.
Copulatory organs are absent, except in the Apoda, in which a portion of the cloaca can be everted and acts as a penis.