As the larva grows older the original bands of cilia become more sinuous, and a second transverse band with small cilia is formed (in the Mediterranean larva) between the previous transverse band and the anus. The water-vascular vesicle is prolonged into two spurs, one on each side of the stomach. A pulsating vesicle or heart is also formed ([fig. 274] B, ht), and arises, according to Spengel (No. [572]), as a thickening of the epidermis. It subsequently becomes enveloped in a pericardium, and is placed in a depression in the water-vascular vesicle. Two pairs of diverticula, one behind the other, grow out (Agassiz, No. [568]) from the gastric region of the alimentary canal. The two parts of each pair form flattened compartments, which together give rise to a complete investment of the adjoining parts of the alimentary tract. The two parts of each coalesce, and thus form a double-walled cylinder round the alimentary tract, but their cavities remain separated by a dorsal and ventral septum.
Eventually (Spengel) the cavity of the anterior cylinder forms the section of the body cavity in the collar of the adult, and that of the posterior ([fig. 274] B, c) the remainder of the body cavity. The septa, separating the two halves of each, remain as dorsal and ventral mesenteries.
The conversion of Tornaria ([fig. 274] A) into Balanoglossus ([fig. 274] B) is effected in a few hours, and consists mainly in certain changes in configuration, and in the disappearance of the longitudinal ciliated band.
The body of the young Balanoglossus ([fig. 274] B) is divided into three regions (1) the proboscidian region, (2) the collar, (3) the trunk proper. The proboscidian region is formed by the elongation of the præ-oral lobe into an oval body with the eye-spots at its extremity, and provided with strong longitudinal muscles. The heart (ht) and water-vascular vesicle lie near its base, but the contractile cord connected with the latter is no longer present. The mouth is placed on the ventral side at the base of the præ-oral lobe, and immediately behind it is the collar. The remainder of the body is more or less conical, and is still girt with the larval transverse ciliated band, which lies in the middle of the gastric region in the Mediterranean species, but in the œsophageal region in the American one.
The whole of the body, including the proboscis, becomes richly ciliated.
One of the most important characters of the adult Balanoglossus consists in the presence of respiratory structures comparable with the vertebrate gill slits. The earliest traces of these structures are distinctly formed while the larva is still in the Tornaria condition, as one pair of pouches from the œsophagus in the Mediterranean species, and four pairs in the American one ([fig. 275], br).
Fig. 275. Late stage in the development of Balanoglossus with four branchial clefts. (After Alex. Agassiz.)
m. mouth; an. anus; br. branchial cleft; ht. heart; W. water-vascular vesicle.