In the Mediterranean Tornaria the two pouches meet the skin dorsally, and in the young Balanoglossus ([fig. 274] B, br) acquire an external opening on the dorsal side. In the American species the first four pouches are without external openings till additional pouches have been formed. Fresh gill pouches continue to be formed both in the American and probably the Mediterranean species, but the conversion of the simple pouches into the complicated gill structure of the adult has only been studied by Agassiz (No. [568]) in the American species. It would seem in the first place that the structure of the adult gill slits is much less complicated in the American than in the Mediterranean species. The simple pouches of the young become fairly numerous. They are at first circular; they then become elliptical, and the dorsal wall of each slit becomes folded; subsequently fresh folds are formed which greatly increase the complexity of the gills. The external openings are not acquired till comparatively late.
Our knowledge of the development of the internal organs, mainly derived from Agassiz, is still imperfect. The vascular system appears early in the form of a dorsal and a ventral vessel, both pointed, and apparently ending blindly at their two extremities. The two spurs of the water-vascular vesicle, which in the Tornaria stage rested upon the stomach, now grow round the œsophagus, and form an anterior vascular ring, which Agassiz describes as becoming connected with the heart, though it still communicates with the exterior by the dorsal pore and seems to become connected with the remainder of the vascular system. According to Spengel (No. [572]) the dorsal vessel becomes connected with the heart, which remains through life in the proboscis: the cavity of the water-vascular vesicle forms the cavity of the proboscis in the adult, and its pore remains as a dorsal (not, as usually stated, ventral) pore leading to the exterior.
The eye-spots disappear.
Tornaria is a very interesting larval form, since it is intermediate in structure between the larva of an Echinoderm and trochosphere type common to the Mollusca, Chætopoda, etc. The shape of the body especially the form of the ventral depression, the character of the longitudinal ciliated band, the structure and derivation of the water-vascular vesicle, and the formation of the walls of the body cavity as gastric diverticula, are all characters which point to a connection with Echinoderm larvæ.
On the other hand the eye-spots at the end of the præ-oral lobe[230], the contractile band passing from the œsophagus to the eye-spots ([fig. 273]), the two posterior bands of cilia, and the terminal anus are all trochosphere characters.
The persistence of the præ-oral lobe as the proboscis is interesting, as tending to shew that Balanoglossus is the surviving representative of a primitive group.
Bibliography.
(567) A. Agassiz. “Tornaria.” Ann. Lyceum Nat. Hist. VIII. New York, 1866.
(568) A. Agassiz. “The History of Balanoglossus and Tornaria.” Mem. Amer. Acad. of Arts and Scien., Vol. IX. 1873.
(569) A. Götte. “Entwicklungsgeschichte d. Comatula Mediterranea.” Archiv für mikr. Anat., Bd. XII., 1876, p. 641.
(570) E. Metschnikoff. “Untersuchungen üb d. Metamorphose, etc. (Tornaria).” Zeit. für wiss. Zool., Bd. XX. 1870.
(571) J. Müller. “Ueb. d. Larven u. Metamor. d. Echinodermen.” Berlin Akad., 1849 and 1850.
(572) J. W. Spengel. “Bau u. Entwicklung von Balanoglossus.” Tagebl. d. Naturf. Vers. München, 1877.
[230] It would be interesting to have further information about the fate of the thickening of epiblast in the vicinity of the eye-spots. The thickening should by rights be the supra-œsophageal ganglion, and it does not seem absolutely impossible that it may give rise to the dorso-median cord in the region of the collar, which constitutes, according to Spengel, the main ganglion of the adult.