As far as is known, all sepiolid embryos form a spine-like integumental structure at the rear of the mantle, just below the hatching gland. The muscular base of this so-called terminal spine can contract independently to stretch out the tough apex of the spine. This stretching movement
is normally superimposed on the contraction of the underlying mantle muscle when the mantle end is thrust against the inner wall of the egg case during hatching. The primary function of the terminal spine can be considered in relation to the presence of a hard surface layer of the egg capsule.
This “shell” resists the dissolving action of the hatching enzyme and must be forced open by the mechanical action of the terminal spine which is pressed against it (pressure being generated by muscular action of the mantle and the arms which are propped against the capsule wall
opposite to the hatch opening). Although this condition is observable only in the genus Rossia (and probably exists in other Rossiinae), it can be considered as the ancestral condition from which the phenotype of the Sepiolinae and the Heteroteuthinae (leathery outer coat of egg capsule
instead of a hard shell) can be derived. The important fact in systematics is the generalized presence of the terminal spine in the Rossiinae and Sepiolinae; whether the hatchlings of the Heteroteuthinae really have this structure remains to be seen.
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