The stomodæum of Petromyzon is surprisingly large, and its size and structure in this type militate against the view of some embryologists that the stomodæum originated from the coalescence of a pair of branchial pouches.
In the region of the trunk there is present an uninterrupted dorsal fin continuous with a ventral fin round the end of the tail.
There is a well-developed body cavity, which is especially dilated in front, in the part which afterwards becomes the pericardium. In this region is placed the nearly straight heart, divided into an auricle and ventricle ([figs. 42] and [43]), the latter continued forwards into a bulbus arteriosus.
The myotomes are now very numerous (about 57, including those of the head, in a three days’ larva). They are separated by septa, but do not fill up the whole space between the septa, and have a peculiar wavy outline. The notochord is provided with a distinct sheath, and below it is placed a subnotochordal rod.
The alimentary canal consists of a narrow anterior section free from yolk, and a posterior region, the walls of which are largely swollen with yolk. The anterior section corresponds to the region of the œsophagus and stomach, but exhibits no distinction of parts. Immediately behind this point the alimentary canal dilates considerably, and on the ventral side is placed the opening of a single large sack, which forms the commencement of the liver. The walls of the hepatic sack are posteriorly united to the yolk-cells. At the region where the hepatic sack opens into the alimentary tract the latter dilates considerably.
The posterior part of the alimentary tract still constitutes a kind of yolk-sack, the ventral wall being enormously thick and formed of several layers of yolk-cells. The dorsal wall is very thin.
The excretory system is composed of two segmental ducts, each connected in front with a well-developed pronephros (head-kidney), with about five ciliated funnels opening into the pericardial region of the body cavity. The segmental ducts in the larvæ open behind into the cloacal section of the alimentary tract.
Fig. 47. Head of a larva of Petromyzon six weeks old. (Altered from Max Schultze.)
au.v. auditory vesicle; op. optic vesicle; ol. olfactory pit; ul. upper lip; ll. lower lip; or.p. papillæ at side of mouth; v. velum; br.s. extra branchial skeleton; 1-7. branchial clefts.