{219}[Illustration: FIG. 114.—Development of _Callidina_. (After Zelinka.) A, Early stage showing involution of granular cells (_g_), to form the mastax or gizzard. B, Involution complete. C, Second involution of epiblast cells to form pharynx. D, The embryo bent on itself at ventral fold (_vf_). E, Showing ingrowth of epiblast to form brain (_br_): _an_, involution of epiblast to form cloaca; _br_, brain; _ep_, epiblast; _fg_, involution to form cement glands of foot; _g_, granular cells; _gi_, gizzard; _hyp_, hypoblast; _m_, mouth; _o_, ovary; _sp_, salivary glands; _vf_, limiting body from foot.] Owing to the elongation of the body within the narrow space of the egg the hinder part is bent up on the ventral surface (D, E); and this part, narrower than the rest, forms the foot, the centre of which is at first occupied by a column of hypoblast. The cloaca is now formed by a dorsal ingrowth of epiblast (the "proctodaeum") at the junction of the foot and the body (_an_). The hypoblast in the body anterior to the cloacal ingrowth forms the digestive apparatus; the part immediately behind forms the reproductive organs (_o_); and the hindmost part apparently disappears. An ingrowth of epiblast at the extreme tip of the foot gives rise to the cement glands (_fg_). The muscles arise from the epiblast cells. The disc arises from the modification of epiblast cells lateral to and behind the mouth, enclosing a so-called "polar area"; it is completed by the transformation of cells on the ventral side of the mouth. The brain (_br_) is formed by the multiplication of epiblast cells; and in Bdelloida a ventral ingrowth below the mouth forms the sub-oesophageal ganglion. The ciliated cup in _Melicerta_ is formed as a ventral hollow, only later on united with the ciliated furrow of the wreath by the lateral grooves.[268] In _Melicerta_ the two eyes are formed in the polar area. The young as hatched {220}differs from the adult in the greater simplicity of its ciliary wreath; and in the tubicolous forms the cupped end of the foot-gland is ciliated, and two eyes are present on the polar area, which later sink in, and often disappear more or less completely. It is stated that the young hatched from winter eggs do not pass through this larval state. [Illustration: FIG. 115.—_Stephanoceros eichhornii._ (After Cubitt.) A, Dorsal view of the upper part in its tube: _al_, lateral antenna; _e_, eye; _em_, developing embryo in uterus; _g_, gizzard; _s.g_, median salivary gland. B, Extremity of foot. C, Lateral view of base of disc: am, median antenna; _of_, oral funnel; _s.g_, median salivary gland; _tf_, in the crop, indicates the ciliated tube prolonging the funnel; _tr_, horseshoe-shaped trochus.] CLASSIFICATION.[269]