(β) No genital wings. .......... Harrimania, Ritter.
The name Balanoglossus was introduced by Delle Chiaje in 1829 for B. clavigerus (Fig. 1, A), from the neighbourhood of Naples. As Spengel has shown, its etymology has been much misunderstood. The second half of the name refers to a fancied resemblance between the Balanoglossus, with its largely developed genital wings, and the tongue of an ox. Βάλανος means "acorn," and it has usually been supposed that this name was suggested by the resemblance of the proboscis, projecting from the collar, to an acorn in its cup, a view which finds its expression in the name "Eichelwurm" used by German zoologists. But the idea expressed by Delle Chiaje was really a similarity between the collar of Balanoglossus and the outer shell of Balanus, the barnacle or "acorn-shell" found everywhere on rocks between tide-marks.
Fig. 8.—Metamorphosis of Balanoglossus, probably of Balanoglossus biminiensis Willey, Bahama Islands. All the figures are magnified to the same scale (× 14). A, fully developed free-swimming larva, or Tornaria, side view; B, commencement of metamorphosis, side view; C, later stage, dorsal view. Increase in size takes place after this stage; a, anus; b.c1, body-cavity of proboscis; c, collar; c.r, transverse ciliated ring; d.p (in A), dorsal pore (= proboscis-pore), seen also in C on the left side, just behind the reference line p.c; e, eyes and sensory thickening of skin (in A); g, gill-pore; g.s, gill-sacs, developing as outgrowths of the alimentary canal; three are already present in B, but are better seen in C, in which they are still without openings to the exterior; l, postoral part of the longitudinal band of cilia; l′, its praeoral part; both l and l′ are produced (in A) into tentacles, over which the band of cilia is looped; the groove in the middle of the figure, between l and l′, conducts the food by the transverse groove to the mouth (m); p.c, blood-space of proboscis and pericardium ("heart" of larva); s, stomach. (After Morgan).
Development.—The free-swimming, larval stage of Balanoglossus is known as Tornaria (Fig. 8, A). Several distinct forms of the larva are known,[[27]] although it is not yet possible to refer them with certainty to their respective adults.
Tornaria was described and named by Johannes Müller, who regarded it as the larva of a Starfish,[[28]] in spite of his intimate knowledge of the development of these animals. Its correct systematic position was first demonstrated by Metschnikoff in 1869.
The larva agrees with many other pelagic forms in being excessively transparent. The form described by Spengel as T. grenacheri attains the remarkable length of 9 mm. (nearly ⅖th inch).
The full-grown larva is usually ovoid, and a complicated "longitudinal" band of cilia runs in several loops over its anterior two-thirds. In side view, part of the surface limited by the ciliated band appears like a T with a double outline, the cross piece being bent downwards on each side, so as to form an anchor-like curve, the middle of which is at the anterior pole of the larva. In T. krohni, which occurs on our south coast,[[29]] the ciliated band has a wavy course. In the West Indian larva[[30]] shown in Fig. 8 A, the ciliated band is produced into numerous tentacles, which fringe the sides of the T-shaped areas or grooves of the surface. These grooves and the cilia which border them are used for conveying food to the mouth.[[31]] At the apex of the larva is a thickening (e) of the ectoderm, bearing two eye-spots. The main locomotor organ is a simple transverse band (c.r) of "membranellae," vibratile structures composed of fused cilia. The mouth (m), on the ventral side, leads into the oesophagus, and this into the stomach (s). The latter is separated by a marked constriction from the intestine, which opens by the anus (a) at the posterior pole.
On the dorsal side is a pore, the "dorsal pore" (d.p.), which leads into a thin-walled sac (b.c1) destined to become the proboscis-cavity of the adult. To the right of the dorsal pore lies the pulsating "heart," which apparently becomes the pericardium of the adult. Bourne and Spengel regard it as a right proboscis-cavity. In the older larvae, the second and third body-cavities appear as paired thin-walled sacs in close contact with the hinder part of the stomach. The skin is very thin, and the five body-cavities do not nearly fill the space between it and the alimentary canal. This space becomes obliterated for the most part by the enlargement of the body-cavities, and its last remains persist, as in many other animals,[[32]] as the vascular spaces of the adult.
In Dolichoglossus kowalevskii, and probably in other species with large eggs,[[33]] development proceeds by gradual stages to the adult form, and no Tornaria-stage is passed through. The opaque young animal, on being hatched, creeps about in the muddy sand in which the adult is found, later moving in a leech-like manner, by alternately attaching itself by its two ends. The young stages were ingeniously obtained by Bateson, to whom our knowledge of the development of this species is due,[[34]] by allowing a large quantity of the mud to settle after being stirred up, the layer of the specific gravity corresponding with that of the young Balanoglossus being then separated by means of a siphon. The young stages previously contained in several hundredweight of mud were thus easily collected into a pint of water. Morgan recommends treating the layer obtained by a similar process with picric acid, which stains the young Balanoglossus yellow.