228 



ICHTHYOLOGY. 



Classifica- bodies, like the Lamprey. The h|)s are furnished with eight 



tion Der- cirrhi, ami above is a spout-hole conimunicating with the 



™ P '^'"'- mouth ; the body is nearly cylindrical, and terminates in a 



^~~^'~^ ' tin which surrounds the tail. The intestine is simple, wide, 



and straight, as viewed externally, but it is plaited within ; 



the liver has two lobes ; the eggs grow to a considerable 



size. When taken and confined in a large glass jar, a single 



fish will pour so much mucus from its lateral pores as to give 



the water the appearance of jelly. 



Ammocre/es. Body cylindrical, with numerous annular 

 lines around it, that give it much tlie appearance of a worm. 

 It lives in the mud of rivers. Mouth incapable of adliering 

 6y suction to other bodies ; fins very shallow ; tail sharp at 

 the tip. The only species is P. branchinlis, Shaw (the Pride 

 of Pennant), which grows to six or eight inches long, and is 

 as thick as a goose-quill. It inhabits the rivers of Oxford- 

 shire, and occurs in various parts of the European continent. 

 Pttromi/zon, Dmn., or Lamprey. The tongvie, acting 

 like a piston in the circular moutli, is capable of vigorous 

 motion, and is an essential part of tlie mechanism by which 

 the fish is enabled to attach itself firmly to stones, or to 

 fasten itself to the larger fishes, which it is thus enabled 

 to suck and devour at its leisure. The dorsal fin is farther 

 forward than the anus, and a second unites with the tail. 

 The European species are, — P. marimis, the Greater Lam- 

 prey, which grows to the length of more than three feet. 

 It is considered as a delicate food, and is caiight as it 

 ascends rivers in the end of winter and spring. Colour 

 yellowish, marbled with brown. First dorsal fin very dis- 

 tinct fi-om the second. This fish is common in the Severn, 

 and in the mouths of many European rivers. Its supposed 

 hermaphroditism is mentioned by Sir Everard Home. — P. 

 fluviatilis, the Lampern, or Nine-eyed Eel. Length 

 fi'om twelve to eighteen inches ; olive back, silvery below ; 

 first dorsal distinct from the second. Two thick teeth, 

 separate, in the top of the maxillary ring. Ascends rivers 

 from the sea ; swarms in the Thames, Severn, and Dee. 

 Vast quantities taken in England are sold to the Dutch for 

 the Turbot fishery. It abounds in the rivers on the southern 

 side of the Baltic. Both these animals are very tenacious 

 of life, and will live many days out of water. — P. planeri. 

 .\bout ten inches long ; greatly resembles the preceding ; 

 but the two dorsal fins are united. It is also an European 

 river fish. Figs. 61, 62 show the dentition of Pitromi/zoit 

 mordar, an Australian species ; the lips and their teeth be- 

 ing sha\cd i)fr in the latter to show the pharyngeal teeth. 



■%^ 



Fij{. 63. 



Fig. 49 represents the lateral gill-openings of the same 

 species, and fig. 63 the entire fish. 



Sub-Ort)ER II.— MARSIPOBRANCHII. 



Cydostomij Dum., Cuv. Gills fixed, bursiform, inoperculate, 

 receiving the respiratory streams by apertures usually numerous 

 and lateral, distinct from the mouth ; a heart. 



These are vermiform fishes whose vertical fins are folds of skin 

 surrounding the tail, and the rays scarcely to be perceived in any 

 part, being soft and rudiraental. They are the first in the ascend- 

 ing series of fishes in which the anterior end of the myelon has 

 that ganijlionic development which constitutes the brain in fishes, 

 and which have a cartilaginous cranium for its protection. In 

 AmmoccEtcs the persistent cranial cartilage resembles the first 

 appearance of the cartilages in the embryo of the higher fishes. h\ 

 the Myxinnids the neural and ha?mal canals of the spinal column 

 are formed of layers of the sheath of the gelatinous dorsal chord ; 

 the neural canal extending along the whole upper part of the chord, 

 and the haemal canal being ccafined to the tail. In the Petromyzons 

 cartilaginous plates are developed in the fibrous sheath, which is 



the first indication of neural arches. A cartilaginous barrel-shaped f'lassifira- 



basket, homologous with the branchial skeleton of osseous fishes, is tion Ver- 



provided for the support of the gills and the canals leading to them, mopteri. 

 This cartilaginous frame-work is termed by Cuvier cota h'ranchi' c ^ y 



ales ; and he observes, that though it is much developed in the Cy- 

 clostomcs, it is scarcely perceptible in the Rays and Sharks. There 

 are no other ribs in this order. 'I'he Marsipobranchs, however, re- 

 semble the Plagiostomes in having an undivided cephalic cartilage, 

 a spout-hole in the head, and a spiral valve in the intestine. In all 

 other respects thoy differ from them, and especinlly in the complete 

 absence of gill-arches, want of jaws, in the total absence of a 

 muscular tunic to the hultus arteriosus, and in the arterial valves be- 

 yond the branchial opening of the ventricle being only two; they 

 differ further in the genital organs of the one sex having no ovi- 

 duct, and in the other no seminal duct. 



The gills of Lampreys and Myxines are little bags, each of which 

 has its jjroper artery ; and its proper orifice in the pharynx through 

 which the water flows in to pass out on the dermal side bv another 

 hole. These purse-shaped bags are called fixed lungs, in contra- 

 distinction to the gills of osseous fishes, whose exterior edges are 

 free and pectinated ; but Professor Owen has observed that if one 

 side of one of the sacs were adherent to the adjacent side of another, 

 and then each bag slit open and detached from the outer integu- 

 ment, a gill would be formed receiving its blood from two arterial 

 branches, and resembling, were it pectinated, the gill of an osseous 

 fish. The genera have been characterized by the number of the 

 exterior gill-openings. 



The teeth in the Marsipobranchs are composed of indurated albu- 

 minous matter, and when macerated, flake off in successive hollow- 

 cones. Agassiz, from investigations into the embryology of fishes, 

 thinks that the Marsipobranchs are not a proper order of fishes, but 

 merely embryonic forms of the more developed Sharks and Kays 

 from which they ought not to be dissevered in our systems. 



Family I.— AMM0C(ETID.E, 



GENt;s I. Ammoc<etes, Dum. Mouth semicircular, edentate, 

 the posterior lip being transverse, cirrhated within ; gill-bags 

 seven, each with an external lateral ajjerture, and receiving streams 

 of water from the oesophagus without a separate trachea. Olfactory 

 organ single, opening in the median line of the dorsal surface of 

 the head, whence a narrow canal, strengthened by cartilaginous 

 rings, runs to the bottom of the skull, and terminates by a valvular 

 opening in front of tlie occipital cartilage. The pituitary lining is 

 plaited longitudinally. 



Agassiz describes a new species, A. horealis. 



Family II.— MYXINID.K. 



Internal branchial openings cesophageal and equal in number to 

 the branchial sacs ; external ones as many, lateral ; or only one 

 common to all the giUs of a side, and then situated on the ventral 

 aspect ; four barbels on the snout, and two on each side of the mouth 

 which is circular; one tooth on the roof of the mouth, two rows f)n 

 the tongue. Eye as lowly orgnnized as in the Lancelet. Naso-pala- 

 tine tube having a valve at its opening on the roof of the mouth. 



Genus I. JIvxine, Lin. ^Gastrobrandms, Bloch.) Saccular 

 gill-bags, six on each side, receiving the streams of water from the 

 cesophagus by as many short tubes. The efferent tubes empty the 

 water into a common lateral canal which terminates on the ventral 

 aspect of the fish at the end of the first third of its length. Be- 

 tween this aperture and its fellow- of the other side, there is a 

 larger opening rather to the left of the mesial line, which admits 

 the water into the oesophagus. 



Genus II. IIeptatkema. Seven lateral branchial apertures. 

 At the extremity of the snout a depressed opening of a canal lead- 

 ing to the gills. Four rostral cirrhi ; as many labial ones. Kyes 

 white lateral points. 



Family IIL— PETROMYZONTID^. 



Branchial organ of each side divided by transverse partitions 

 into seven fixed chambers, which receive tlie water from a median 

 canal, distinct from the cesophagus, by as many round holes, and 

 permit it to flow out by an equal number of lateral cutaneous ori- 

 fices ; the median canal lies beneath the oesophagus, opens anteriorly 

 into the fjiuces by a doubly valvular orifice, communicates with 

 the gill-chambers to the right and left, and is closed at the pos- 

 terior end. Nostril single; tubular on the top of the head, leading 

 to a globular nasal sac from a tube that descends to the base of the 

 skull and the membrane of the palate, which close? it, leaving no 

 perforation there. Cuvier compares this tube to a trachea. Jlouth 

 variously armed with teeth. 



Genus I. Peti;omyzon, auctorum. 



Dr Gray has recently proposed a new arrangement of the Lam- 

 preys. 



