Development.—This has only been fully studied in the summer egg; in Brachionus by Salensky,[[263]] in Melicerta by Joliet[[264]]; in Eosphora digitata and several other species by Tessin[[265]]; in Callidina and Melicerta by Zelinka,[[266]] the last two observers having utilised modern methods of research.[[267]] We shall base our account on Zelinka's observations. As in the case of most "parthenogenetic" eggs, the ovarian egg begins by a very uneven division to form two cells: the minute "first polar body" which undergoes no further development; and the definitive egg, which by its repeated divisions gives rise to the tissues and organs.
Segmentation is very unequal, and recalls that of Molluscs in several respects. The first division gives rise to a smaller and a larger cell. Both of these divide again, the latter unequally, so that now there are three smaller cells and one large one; and after repeated divisions of the small cells and unequal divisions of the larger one, a stage is reached where there are a number of small cells and one large one, which sinks in and is overgrown by the small ones. Just prior to this the large cell undergoes equal divisions; its cells are the "hypoblast" cells (Fig. 114, hyp), and give rise to the gullet, stomach, and intestine, with their appendages, and the generative organs; while the smaller cells constitute the "epiblast" (ep), which gives rise to the body-wall and muscles, to the cement glands, nervous system, pharynx and mastax, and probably to the kidneys.
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 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.
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]]
Order I. Flosculariaceae.—Females mostly tubicolous, attached by a long contractile foot. Disc produced into a wide funnel-shaped contractile cup, produced into lobes with long setae (Floscularia) or coarse cilia (Stephanoceros), or entire (Apsilidae); an outer row of fine cilia rarely present; trochus a horseshoe, open behind. Oral funnel a slender tube hanging freely into a large pharyngeal crop; trophi uncinate projecting freely into the crop. Kidneys often united by an anterior cross-piece. Body-wall often containing a definite system of canals, filled with refractive granules, and serving by their contraction to dilate the disc. Males (Fig. 107, 1) and larvae vermiform with a ciliated pedal cup, and a simple wreath, with two eyes on the disc.
Fam. 1. Flosculariidae: Floscularia Oken, Stephanoceros E. (Fig. 115).