. The anatomy of the common squid : Loligo pealii, Lesueur. Loligo pealii; Mollusks -- Anatomy. indicated (text figure 2) very early by the appearance of a pit, tlie beginning of tfie shell gland, around which the mantle ridge forms. The oral invagination and the anlage of the cephalic ganglion mark the anterior end of the embryo. The large head-folds, partly surrounding the optic pits, form the greater portion of the sides of the embryo. The statocyst, the anlagen of the siphon, and of the gills lie on the posterior surface. The foot is represented by a band of tissue which connects the embry
. The anatomy of the common squid : Loligo pealii, Lesueur. Loligo pealii; Mollusks -- Anatomy. indicated (text figure 2) very early by the appearance of a pit, tlie beginning of tfie shell gland, around which the mantle ridge forms. The oral invagination and the anlage of the cephalic ganglion mark the anterior end of the embryo. The large head-folds, partly surrounding the optic pits, form the greater portion of the sides of the embryo. The statocyst, the anlagen of the siphon, and of the gills lie on the posterior surface. The foot is represented by a band of tissue which connects the embryo with the yolk and from which five pairs of elevations, the anlagen of the five pairs of arms, project. The anus has not yet formed but its future position is indicated by the mesenteron which lies beneath the ectoderm of the back of the embryo. The essential relations to be noted are, that the foot is ventral, the mouth anterior, and the shell-gland dorsal. A more ca advanced embryo is represented by A-e text figure 3. The convex visceral ^<^ dome of the younger embryo has _Gi become conical and at the same "*" time the ventral portion of the embryo S' has contracted constricting the yolk- ^. sac from the embryo. The mantle has grown downward so that it now covers the gills and the newly formed anus. The gut is complete and near its middle, makes an angle from which two evaginations, the undeveloped stomach and caecum, project upward. The heart is dorsal to the gut. The statocysts, two pairs of siphonal folds, the pedal, and the visceral ganglia are on the posterior side of the embryo between the mantle and the arms. This change may be summed up by saying that the dorso-ventral axis has elongated while the sagittal and transverse axes have become relatively short. This process continues in the Decapoda beyond the stage represented in text figure 3 until the adult form (text figure 4) is reached. In the adult Decapod the dorsal and ventral surfaces are reduced to
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