that the olfactory organ becomes sunk down beneath the surface through becoming involved in the ectodermal ingrowth which forms the pituitary tube. As a further consequence in the case of the lampreys the olfactory organ becomes transported to the roof of the head along with the pituitary opening, which latter functions as an external nostril. That the unpaired olfactory organ of existing Cyclostomes has passed through, in their ancestors, a paired condition such as exists in other vertebrates, is indicated by the fact that it retains a pair of olfactory nerves.
The eyes in adult lampreys are of moderate size, while in the Myxinoids they are greatly reduced—sunk beneath the skin (Bdellostoma) or even in amongst the muscles of the head (Myxine). The lens is completely absent, also the ocular muscles. The otocyst or auditory organ is unique amongst craniate vertebrates in regard to the semicircular canals. In the lampreys there are only two instead of the normal three, while the Myxinoids have only one.
Alimentary Canal.—The widely gaping buccal funnel is morphologically an inpushing of the outer skin, i.e. it is stomodaeal in nature. The thorn-like teeth which stud its lining are formed simply by cornification of the epidermal cells (4) like the provisional horny teeth of a tadpole, and are not homologous with the true teeth of ordinary vertebrates. As to whether they represent the remnant of a once present system of epidermal scales, which may have preceded the coating of placoid elements in the evolution of the vertebrate, there is no evidence.
 | |||
| Modified from T. J. Parker, Zootomy, fig. 4, by permission of Macmillan & Co., Ltd. | |||
| Fig. 2.—Median longitudinal section through anterior end of Petromyzon. | |||
| a.v.o, | Atrio-ventricular opening. | oes, | Oesophagus. |
| br, | Brain. | olf, | Olfactory organ. |
| br.o, | Internal opening of gill sac. | pc, | Pericardium. |
| d.a, | Dorsal aorta. | p.c.v, | Left posterior cardinal vein. |
| d.c, | Ductus cuvieri. | pit, | Pituitary tube. |
| h.v, | Hepatic vein. | V, | Ventricle. |
| i.j.v, | Inferior jugular vein. | v, | Velum. |
| N, | Notochord. | ||
The pharyngeal region, closely associated with the respiratory function, possesses, on each side, a series of gill-sacs (six in Myxine: seven in Petromyzon, besides an anterior one which is laid down in the embryo but disappears later: up to as many as fourteen in Bdellostoma) opening on the one hand to the pharynx and on the other to the exterior. In Bdellostoma and in the larva of Petromyzon the gill-sacs open directly from the pharynx to the exterior, but in the adult lamprey and in Myxine the original relations are modified. In Myxine, the external openings of the gill-sacs have migrated backwards along the side of the body and become coincident at a point slightly posterior to the last sac. It follows from this that each sac is connected with the common aperture by a tube, longest in the case of the first sac, shortest in the case of the last. In the adult lamprey a different modification is found. Here the dorsal portion of the pharynx has become nipped off as a narrow tube which functions as an oesophagus from the larger ventral portion, which forms an elongated saccular structure ending blindly at its hinder end and having in its lateral wall the internal openings of the gill-sacs.
Breathing.—The inspiratory current passes inwards by the mouth opening in the larval lamprey, by the pituitary tube in Myxine, while in the adult lamprey both expiration and inspiration takes place through the external gill-openings. In the case of the lampreys the elastic skeleton of the branchial region (see below) plays an important part in respiration. The branchial region shows rhythmic contraction through the agency of the transverse muscles—and expansion, through the elasticity of the branchial skeleton—in the adult lamprey. These rhythmic movements of the branchial region cause successive inflow and outflow through the branchial openings. In the larva, on the other hand, the respiratory current always passes in one direction—backwards. This is helped by the presence of a velar fold at the front end of the pharynx, which acts as a valve opening only backwards, and to the presence of membranous flaps projecting back from the anterior border of each gill-opening and acting as valves which open only outwards.
Behind the pharynx comes the truly digestive part of the alimentary canal in the form of a straight tube showing little differentiation into special regions. The lining of the intestine is increased in area by an inwardly projecting fold, which is compared by some morphologists with the spiral valve of certain other groups. In the mature river lamprey the digestive tract becomes in great part degenerate.
Coelomic Organs.—The chief point of interest about the splanchnocoele or perivisceral cavity is that in the Myxinoids the adult shows a persistent embryonic condition in that the pericardiac portion never becomes isolated from the main body cavity.
The renal organs are of special interest in the Myxinoids from their very simple character. The kidney duct is seen running along the roof of the coelom on either side. Into the duct open short segmentally arranged tubes, each possessing at its closed rounded extremity a Malpighian body. Each of these short tubes is morphologically a nephric tubule, which, however, in correlation with its shortness, is without the turns and twists so characteristic of such tubules generally. A further consequence of the short simple character of the tubules is that they are quite separate from one another, instead of being massed together to form a compact gland such as the kidney is elsewhere. In Petromyzon the kidney has the ordinary compact form, and here also the Malpighian bodies are shut off from the splanchnocoele.
The ovary or testis is a large unpaired structure hanging from the dorsal wall of the splanchnocoele and shedding its products into it; from the coelomic space the genital products pass into the urogenital sinus—formed by the fusion of the kidney ducts at their hinder ends—through a small opening, one at each side. This opening, which leads directly from coelom into urogenital sinus, is known as the genital pore. Its morphological significance is doubtful.
Skeleton.—The vertebral column of the lamprey is represented by a persistent notochord surrounded by a thick sheath, which shows no signs of invasion by cartilage cells or of segmentation. Resting on the sheath are paired dorsal arch elements, more numerous than the neuromuscular segments. In the tail region these are united into a continuous band of cartilage on each side: similar cartilaginous bands represent the ventral arch elements of the tail region. The skeleton of the head region consists of a cartilaginous cranium, into the formation of which enter typical parachordal and trabecular elements, together with olfactory and auditory capsules. In addition to these, there are a number of other cartilaginous pieces present in the head region, the homologies of which are doubtful.
Branchial Basket.—One of the most characteristic features of the skeleton of the lamprey is the remarkable cartilaginous “branchial basket,” which supports the gill region. In an adult river lamprey the basketwork consists on each side of a series of eight vertical half-hoops of cartilage. The hoops of each side are connected together dorsally by a pair of longitudinal bars, lying ventral to the notochord, and ventrally by a similar pair of rods which are fused in the middle line. Slender cartilaginous projections arise from the anterior and posterior sides of the hoops, and certain of these meeting at their ends form additional longitudinal bars connecting together successive hoops. Connected with the basketwork posteriorly is a remarkable cup-shaped cartilage, which supports the hind wall of the pericardium. The series of cartilaginous half-hoops naturally suggest the half-hoops of cartilage which form the skeleton of the visceral arches in the Gnathostomata. They are, however, more superficial in position, and this has led many to doubt their actual homology with the cartilaginous visceral arches. Taking into account, however, our present knowledge of the development of the two sets of structures, it seems on the whole probable that a true homology exists and that the branchial basket of the lamprey represents merely a set of visceral arches modified in accordance with the peculiar breathing methods of the creature. In the Myxinoids the branchial basket is reduced to a few vestigial masses of cartilage.
Vascular System.—The heart (5) of the lamprey consists of an atrium and a single ventricle, the atrium on the left, the ventricle on the right. Into the atrium, on its right side, and behind the atrio-ventricular opening, there opens a nearly vertical chamber usually termed the sinus venosus (see below), the opening guarded by a pair of vertically placed valves. The ventricle passes anteriorly into what is clearly the homologue of the conus arteriosus of other forms. In its interior are present a pair of laterally placed longitudinal ridges similar to the ridges which occur in other forms in the conus. The opening from ventricle into conus is guarded by a pair of laterally placed pocket valves situated just within the boundary of the ventricle.
The arterial system is of the ordinary piscine type. From the heart there passes forwards a ventral aorta, split into two separate vessels in its anterior half, and giving off on each side a series of efferent vessels to the gill-sacs, one passing between each two gill-sacs and an additional one to the front wall of the front sac and to the posterior wall of the last. The blood is collected from the walls of the gill-sacs by a series of efferent vessels which open into the dorsal aorta. It is to be noted that the dorsal aorta retains the probably primitive unpaired condition, except for a very short extent at its anterior end, where it is split so as to form two short aortic roots.
Venous System.—The main venous channels are like those in other fishes, though their connexion with the heart becomes modified in the adult. The two posterior cardinals—with their continuations forwards, the anterior cardinals—approach the median plane and undergo fusion in the region of their opening into the two ductus Cuvieri. The left ductus Cuvieri then atrophies so that all the blood from the cardinals reaches the heart by way of the originally right
