Deumopteres.- 



-FISIIES.- 



-Ampiiyo.xids. 



109 



or mere albuminous concretions, wanting the structure 

 of cartilage or of bone. Tliey have no air-bladder, 

 and no pancreatic apparatus to assist in digestion. 

 Their jaws also differ from those of other fishes, having 

 more the character of pliaryngeal arches than of maxil- 

 laries and mandibles. Tliey are divided by Professor 

 Owon into two sub-orders : I. the Pharyvgohranchs or 

 Cin-hosloincs, of which the Lanoelets are the only 

 representatives ; and II. the ]S[arsix>ohranchs or Cyclos- 

 tomes, embracing the families of the Myxinoids or 

 Hags and the Petromyzontids or Lampreys. 



Family I.— AMrilYOXIDS. 



THE LANCELET {Ampliiu.xjcs lanccolatus or BrancJi- 

 iostoina luhricum) was first detected on the Cornish 

 coast, and a specimen being sent to Pallas, tl-.e cele- 

 brated naturalist of Russia, was described by him as a 

 sing. Tlie Cornish ichthyologist, Jonathan Couch, 

 Esq., of Polpcrro, rediscovered it in 1831 on the same 

 coast where it was first found, and many examples have 

 since been obtained on the English, Scottish, Irish, 

 Norwegian, Mediterranean, and West Indian coasts, 

 principally by dredging in water of various depths down 

 to fifty fathoms. Tlie same, or a closely resembling 

 species, has been brought from China, so that this 

 embryonic type of a vertebral is wide'y diffused in the 

 ocean. 



One is not surprised that when modern zoology was 

 in its infancj', this curious creature was taken for a 

 mollusc, and that it was left for the late Mr. Yarrell 

 to detect the nervous axis and establish its piscine 

 character, since it h.as no head, being as its generic 

 name imparts sharp at both ends, no gills, no heart, no 

 stomach distinct from the small gut, and no other fins 

 than a narrow border of skin above and below. It is 

 a thin, transparent animal, destitute of that swelling of 

 the proximal end of the spinal marrow (or myelon) 

 which is called the brain. As the brain of fishes, 

 though composed of several pairs of tubercles of ner- 

 vous substance, and of some single mesial ones, is 

 small in comparison of the cranial cavity in which it is 

 lodged, it is to the absence of the skull, the jaws, the 

 hyoid apparatus, and the other parts connected there- 

 with, that the sharpness of the proximal end of the 

 Lancclet is to be attributed. The vertebral chord con- 

 sists of a series of discoid cells closely pressed together 

 in a fibrous sheath, whose upper layers separate and 

 form a tube for lodging the myelon ; and in like manner 

 the lower layers of the sheath form behind the vent a 

 ha:mal canal, no cartilaginous deposits being traceable 

 in cither. Very simple is the neural axis itself, as it 

 exhibits none of the fibrous or tubular structure which 

 exists in the brain and myelon of the higher vertebrals, 

 but is composed solely of isolated cells, forming a ner- 

 vous tract, which is riband -shaped in the middle third, 

 and a little more round towards the ends. In its course 

 it gives off about sixty pairs of nerves, which spring 

 from single roots. 



The entire nervous system of the Lancelet is so 

 simple, that Professor Owen considers it to be typified 

 by that of the entozoons or intestinal worms. The 

 eye is a mere speck, which some able anatomists have 



failed to detect ; and as it is not furnished with muscles 

 for motion, vision can scarcely' extend beyond the mere 

 perception of light. The animal, however, is very 

 active, and when disturbed speedily hides itself among 

 sand or gravel. 



The length of this fish is generally under two inches; 

 and its longitudinal mouth, situated under the proximal 

 acute extremity, is fringed by feelers supported by the 

 only parts of the skeleton that have the consistence of 

 cartilage. The cavity of the mouth and the commence- 

 ment of the digestive canal are of great comparative 

 dimensions, since they make up half the bidk and length 

 of the fish. The pharynx is a kind of barrel-shaped 

 cage, with more than a hundred vertical slits beset 

 with vibratile ciliao, and opening into the cavity of the 

 abdomen in which the cage is suspended. A great 

 dorsal and ventral blood-vessel encircles the interior 

 of the animal longitudinally, and sends numerous 

 branches to the pharyngeal cage, which thus serves 

 both for the passage of the aliment, and for tlie oxy- 

 genation of the blood, through exposure to the air 

 contained in the water that flows throvigh it. Tlie 

 alimentary particles pass into a simjile intestinal canal 

 issuing from the distal end of the jiharyngeal cage near 

 its dorsal aspect ; and the water of respiration escaping 

 through the lateral ciliated slits into the cavity of the 

 belly, finally issues by an external ventral opening 

 situated between the intestinal vent and the mouth. 

 Through this single ventral gill-opening the eggs of the 

 animal are also extruded. In animals low in the scale, 

 we find a single viscus performing two or more func- 

 tions for which special organs are assigned in animals 

 of a higher order. Duges ascertained that during the 

 first hour of a tadpole's existence, the blood is aerated 

 solely through the medium of the skin, and that a 

 special organ of respiration is not developed until the 

 second or third hour after the animal has made its 

 escape from the egg. Moreover, in several osseous 

 fishes the digestive canal acts an auxiliary part in the 

 oxygenation of the blood. The misgurn loach (cuhilis 

 fossiiis), for instance, swallows atmospheric air by the 

 mouth, and discharges azote through the vent after the 

 oxygen has been consumed in its passage through the 

 intestinal tube, while the gills at tlie same time exhale 

 much carbonic acid.* Except that it is not aided by 

 gills, the greatly developed pharynx of the Lancelet 

 has a similar function. Owen, however, considers 

 some fringe-like processes of membrane which float 

 in the cavity of the mouth of the Lancelet to be " free 

 inoperculate gills." 



Family II.— MYXIXOIDS.- (Plate 1, fig. 1.) 



The term IMarsipobranch apjilied to the second sub- 

 order of Dermopteres, means "gills in pouches;" while 

 the other appellation of Cyclostome used by Cuvier, 

 has reference to the circular suctori.al mouths wliieli 

 all the adult members of the group possess, and which 

 is in firct an essential part of their structure, since all 

 these fishes after passing their larval stage are parasites, 

 and live by sucking in the fluid and soft parts of the 



• Eisclioff; Eiuimcrt, &c., quoted l.y Milnc-Edmrds, Pbys. 



