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  Section: Anatomy of Vertebrate Animals » The Provinces of the Vertebrata - The Class Pisces
 
 
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The Marsipobranchii

 
     
 

In this order of the class Pisces the integument is devoid of scales or bony plates.

The spinal column consists of a thick persistent notochord enveloped in a sheath, but devoid of vertebral centra. The neural arches and the ribs may be represented by cartilages, and there is a distinct skull presenting cartilage at least in its base, and retaining many of the characters of the foetal cranium of the higher Vertebrata. The notochord terminates in a point in the base of this cartilaginous skull behind the pituitary body; and the skull is not movable upon the spinal column. There are no jaws; but the palatopterygoid, the quadrate, the hyomandibular, and the hyoidean apparatus of higher Vertebrata, are imperfectly represented (Fig. 30, f, g, h). In some genera a basket-like cartilaginous apparatus strengthens the walls of the oral cavity; while, in others, such a framework supports the gill-sacs.

The Marsipobranchii possess neither the pectoral nor the pelvic pair of limbs, nor their arches. Horny teeth may be developed upon the roof of the palate, or upon the tongue, or may be supported by peculiarly developed labial cartilages.


A, the skull of a Lamprey, viewed from the side; B, from above:-a, the ethmovo merine plate; b, the olfactory
Fig 30. - A, the skull of a Lamprey, viewed from the side; B, from above:-a, the ethmovo merine plate; b, the olfactory
capsule; c, the auditory capsule; d, the neural arches of the spinal column; e, the palatopterygold portion; f, probably,
the metapterygoid. or superior quadrate, portion, and g, the inferior quadrate portion, of the subocular arch; h, stylohyal process; i, lingual cartilage; k, inferior, l lateral. prolongation of the cranial cartilage; 1, 2, 8, accessory labial cartilages; m, branchial skeleton. The spaces on either side of 1 are closed by niembrane.
The alimentary canal is simple and straight, and the liver is not sac-like, but resembles that organ ia other Vertebrata.

The heart has the usual piscine structure, consisting of a single auricle preceded by a venous sinus, a single ventricle, and an aortic bulb, all separated from one another by valves. This heart is contained in a pericardium, the cavity of which communicates with that of the peritonaeum.

In Myxine the portal vein is rhythmically contractile.
The cardiac aorta, which is continued from the bulb, distributes its branches to the respiratory organs. These consist of antero-posteriorly flattened sacs, which communicate directly or indirectly, on the inner side, with the pharynx, and, externally, with the surrounding medium.

In the Lamprey there are seven sacs, upon each side, which open externally by as many distinct apertures. Internally, they communicate with a long canal, which lies beneath the oesophagus and is closed behind, while anteriorly it communicates freely with the cavity of the mouth (Fig. 33, Pr).

Side and upper view of the brain of Petromyzon fluviatilis, and an
Fig. 31. - Side and upper view of the brain of Petromyzon fluviatilis, and an
upper and inner view of the membranous labyrinth of P. marinus. The following letters refer to the flgnres of the brain: I., the olfactory nerves, narrow anterior prolongations of the rhinencephalon (A); B, the prosencephalon; C, the thalamencephalon; D, the mesencephalon; E, the medulla oblongata; F, the fourth ventricle; e, the narrow band which is all that represents the cerebellum; G, the spinal cord; II., theoptic; III., the oculomotorius; IV., the patheticus; V., the trigeminal; VI., the abducens; VII., the facial, and the auditory; VIII., the glosso- pharyngeal and pneumogastric; IX., the hypoglossal nerves; 1, 1', 2, 2', sensory and motor roots of the first two spinal nerves. In the figure of the membranous lahyrinth: k, the auditory nerve; a, the vestibule; c, the two semicircular canals, which correspond with the anterior and posterior vertical canals of other ertebrata; d, their union and common opening into the vestibule; b, the ampullae.
The kidneys are well developed, and have the ordinary vertebrate structure, while the ureters open behind the rectum.

The brain, though very small, is quite distinct from the myelon, and presents all the great divisions found in the higher Vertebrata—that is to say, a fore-brain, mid-brain, and hind-brain. The fore-brain is further divided into rhinencephala, solid prosencephalic lobes, and a thalamencephalon; the hind-brain, into metencephalon and myelencephalon (Fig. 31).

The auditory organ is simpler than in other fishes, possessing only two semicircular canals and a sacculated vestibule in the Lamprey. In Myxine the whole organ is represented by a single circular membranous tube, without further distinction into canals and vestibule.

The Marsipobranchii differ remarkably, not only from the fishes which lie above them, but from all other vertebrate animals, in the characters of the olfactory organ, which consists of a sac placed in the middle line of the head, and having a single, median, external aperture. In all other Vertebrata there are two nasal sacs. In the Lampreys, the nasal sac terminates blindly belovr and behind, but in the Hags (Myxine), it opens into the pharynx. In no other fishes, except Lepidosiren, does the olfactory apparatus communicate with the cavity of the mouth.

The reproductive organs of the Marsipobranchii are solid plates suspended beneath the spinal column, and Ihey have no ducts, but shed their contents into the abdomen, whence they pass out by an abdominal pore. In the early stages of their development the Lampreys present some singular resemblances to the Amphibia. They also undergo a metamorphosis, the young Petromyzon being so unlike the parent, that it was, until lately, regarded as a distinct genus-Ammocoetes. But the young Lampreys never possess external branchial filaments or spiracula.

The Marsipobranchii are inhabitants of both fresh and salt water. The Myxinoids are remarkable for their parasitic habits the Hag boring its way into the bodies of other fishes, such as the Cod. No fossil Marsipobranchii are known. This circumstance may, in part, be due to the perishableness of their bodies; though horny teeth, like those of the Lampreys, might have been preserved under favorable circumstances.


 
     
 
 
     



     
 
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