The gland tubes develope into a network surrounded by a vascular network, and their tubular character, retained in Amphibia and Reptilia, is lost in Aves and Mammalia by the growth of the lining epithelium. The number of bile ducts vary, and there is frequently a gallbladder or caecum attached to one of them. The fully formed gland is typically bilobed, but its external shape is subject to great changes. The pancreas arises as a single, or in Aves double, dorsal outgrowth of the mesenteron into the mesoblast. It is eventually closely connected (with few exceptions) to the bile duct near its intestinal opening. It is absent in some Pisces.
A diverticulum from the oesophagus, wanting in Elasmobranchii and Holocephali, becomes the air-bladder of Pisces, the lungs of the higher Ver-tebrata. It is primitively ventral in position, dorsal in nearly all Pisces in which the original communication with the oesophagus is often lost. It is either single or double, but in the case of the lungs always becomes double, remaining attached by a stalk to the oesophagus. The stalk is the trachea, the two appended caeca the bronchi and lungs. In Aves and Mammalia the caeca branch repeatedly forming the bronchial, and the ultimate branches - the air-tubes or cells. Certain of the bronchia are prolonged into air-sacs in Aves. The hypoblast cells form the epithelium lining the whole structure, the surrounding mesoblast, the supporting and vascular tissues. The trachea, bronchi, and the bronchia when present, are strengthened by cartilaginous rings or pieces most complete in the two structures first named. The first rings of the trachea differentiate into a larynx, with which are connected the fibrous bands or cords which produce the voice. The organ of voice, however, in Aves is developed at the junction of the trachea and bronchi, and is known as the syrinx.
A valve, the epiglottis, covers the entrance to the larynx in Mammalia.
Two other structures, the thyroid and thymus, are closely connected to the fore-part of the alimentary canal. The former develops as a ventral diverticulum of the mouth, lying at the anterior end of the ventral aorta, and is probably homologous with the endostyle of Urochorda. It closes off from the mouth, and when fully formed is a solid body either single, bilobed, or broken up into two parts (Amphibia) or many masses (Teleostei)2. The thymus is formed by epithelial growths from the dorsal ends of more or fewer of the branchial (visceral) clefts. These growths fuse together on the right and left sides into a single body. The thymus atrophies in the higher Vertebrata as a rule.
1 The bronchia, which spring from a spot above, i. e. anterior to the point, where the pulmonary artery crosses the bronchus, are known as ep-arterial; those below, or posterior to it, as hyp-arterial.
2 See a paper by Dohrn on the thyroid in Petromyzon, Amphioxus, and Tunicata ( = Urochorda), Mitth. Zool. Stat. Naples, vi. 1885.
There is a closed circulatory system formed in the mesoblast consisting of a central rhythmically contractile heart, a set of efferent vessels or arteries, and of afferent vessels or veins - the two sets connected by the smallest vessels or capillaries. The heart is primitively a specially developed section of the subintestinal vein in the region of the throat, but in Amniote Vertebrata (? Reptilia) it is formed by the coalescence of two vessels directly continuous with the vitelline veins. It consists primitively of a sinus venosus, lost in adult Aves and Mammalia, an auricle and a ventricle separated by valves; a truncus arteriosus divisible into a conus arteriosus, containing many valves and a bulbus arteriosus (or aortae), which is valveless. The auricle lies dorsally to the ventricle and truncus, and anteriorly to the former. It is divided by a septum into a right and left auricle from Amphibia upwards. The ventricle is similarly divided in Aves and Mammalia. The complete division of the ventricle which exists in Crocodilia is perhaps not homologous with that of higher Vertebrata. The conus is found in all Ichthyopsida, except Teleostei, in which it aborts; the bulbus only in Pisces1-. The embryonic truncus (i.e. conus + bulbus) is broken up by an internal septum in all Sauropsida and Mammalia into the roots of the great vessels, i. e. the aorta and pulmonary artery.
The roots of these vessels are guarded in Sauropsida by two, in Mammalia by three, semilunar valves. The heart lies in a special section of the coelome - the pericardium. The part of the coelome dorsal to the pericardium is aborted in Pisces. In other Vertebrata it enlarges, and the lungs grow out into it. The bulbus is primitively continued forwards as the ventral aorta as far as Meckel's arch, and gives off to the right and left paired aortic arches, which correspond to Meckel's arch, the hyoid and following branchial arches. The mandibular aortic arch is aborted in growth, and is sometimes not developed (Amphibia); the hyoidean persists in some Pisces. In Sauropsida and Mammalia only three branchial aortic arches are ever developed, but four at least in most Ichthyopsida. Some or all of these aortic branchial arches, and in some instances the hyoidean also, supply the internal or external gills of Ichthyopsida, in the former case being resolved into a branchial artery and vein. The dorsal ends of the primitive aortic arches unite on each side into a vessel which gives off forwards to the head one or two branches, the carotid arteries, external as well as internal, and then runs backwards beneath the notochord parallel to its fellow with which however it fuses, sooner or later, to form the subvertebral aorta of the adult.
This aorta gives off vessels to the fore-limbs, or the subclavian arteries; a vessel to the yolk sac, the vitelline artery (the persistent mesenteric artery); one or two iliac arteries to the hind-limbs which are connected with the allantois in foetal Sauropsida and Mammalia, by the umbilical arteries, the remnants of which constitute the hypogastric arteries at a later period. The aorta ends in a caudal artery.
1 The distal rows of valves of the conus persist in Teleostei, the proximal apparently, in Sauropsida and Mammalia. The Amphibia have both a distal and proximal row in their conus.