Fam. 2. Apsilidae: Apsilus Metschnikoff (Fig. 112, A), Acyclus Leidy, Atrochus Wierzejski (Fig. 112, C).
The family Flosculariidae contains some most exquisite forms; Stephanoceros, the "Crown Animalcule," being probably the most lovely of the Class, and many of the Floscules coming not far behind. The Apsilidae are mostly mud-dwellers.
Order II. Melicertaceae.—Females (except in Trochosphaera) attached or tubicolous; tube variable. Disc with a dorsal gap (except Conochilus) often two-lobed or corolla-like; a ventral lip often separating off a ventral ciliated cup continuous by a pair of gutters with the ciliated groove; trochus of stronger cilia than the cingulum. Trophi malleoramate in a distinct mastax. Intestine much curved dorsally, cloaca long eversible (except Trochosphaera). Males and larvae as in Order I.
Fig. 116.—Melicerta ringens. (After Joliet). A, Side view; B, dorsal view. al, Lateral antennae; ci, cingulum seen by transparency; g, gizzard; p, pellet in ciliated cup, about to be deposited on edge of tube; tr, trochus.
Fam. 3. Melicertidae: Melicerta E. (Fig. 116), Limnias Schrank, Cephalosiphon E., Oecistes E., Lacinularia E., Megalotrocha E., Conochilus E., Octotrocha Thorpe.
Fam. 4. Trochosphaeridae: Trochosphaera Semper (Fig. 118, D).
The Melicertidae embrace a large number of tubicolous forms, many of which are social. This habit is especially noticeable in Lacinularia socialis, which forms a gelatinous incrustation easily seen by the naked eye; and in Conochilus volvox, which forms free-swimming globular aggregates, the young attaching themselves when hatched to the centre of the ball, and the ball splitting up into two as soon as undue pressure is exerted at the periphery by overcrowding. In this genus the eyes are very conspicuous in the adult, as they are in the similar free-swimming aggregates of Lacinularia racemovata.
Trochosphaera (Fig. 118, D) is remarkable for its peculiar spherical shape, the absence of a foot, the limitation of the viscera to the lower hemisphere, and the dorsal position of the ovary. But a reference to the figure will show that the outgrowth of a foot in the quadrant between the mouth and anus and the flattening of the upper hemisphere would bring its organs on the whole into close correspondence with those of the rest of the Order. It is recorded from South China, the Philippines, and North-East Australia, and has only been seen by Semper, the founder of the genus, and by Thorpe, who saw the male of the first species, and described a second.[[270]]
Order III. Bdelloida.—Females creeping like a leech, as well as swimming (males unknown), susceptible of desiccation and revival ("anabiotic"). Body telescopic at both ends. Disc (except in Adineta) chiefly composed of two dorsal lobes like kettle-drums, wholly retractile; a dorsal proboscis or trunk-like prolongation of the body ends in a ciliated, sensory, and adhesive cup used in crawling, and overhung by a pair of membranous flaps. Trophi ramate; brain with a ventral ganglion, forming a complete ring. Eyes, two on the proboscis or brain, or absent. Bladder a mere dilatation of the rectum. Foot often possessing blind spurs, as well as two or three retractile perforated toes, or forming a terminal disc perforated by numerous pores of the cement glands, rarely ciliated.[[271]]