peculiar importance. The mouth is usually furnished with a complicated series of teeth, which, in the Bony Fishes, are not only developed upon the jaws proper, but are also situated upon other bones which enter into the composition of the buccal cavity (such as the palate, the pterygoids, vomer, branchial arches, the glossohyal bone, &c.) The oesophagus is usually short and capacious, and generally opens into a large and well-marked stomach. The pyloric aperture of the stomach is usually furnished with a valve, and behind it there is usually a number (from one to sixty) of blind appendages, termed the "pyloric cæca." These are believed to represent the pancreas, but there may be a recognisable pancreas either alone or in addition to the pyloric cæca. The intestinal canal is a longer or shorter, more or less convoluted tube, the absorbing surface of which, in certain fishes, is largely increased by a spiral reduplicature of the mucous membrane, which winds like a screw in close turns from the pylorus to the anus. The liver is usually large, soft, and oily, and a gall-bladder is almost universally present; but in the Amphioxus the liver is doubtfully represented by a hollow sac-like organ.

The kidneys of fishes are usually of great size, and form two elongated organs, which are situated beneath the spine, and extend along the whole length of the abdominal cavity. The ureters often dilate, and form a species of bladder, the doubtful representative of the allantois.

Whilst the respiration of all fishes is truly aquatic, most of them are, nevertheless, furnished with an organ which has been generally believed to be the homologue of the lungs of the air-breathing Vertebrates. This the "air" or "swim bladder "—is a sac containing gas, situated beneath the alimentary tube, and often communicating with the gullet by a duct. In the great majority of fishes the functions of the airbladder are certainly hydrostatic-that is to say, it serves to maintain the necessary accordance between the specific gravity of the fish and that of the surrounding water. In the singular Mud-fishes, however, it acts as a respiratory organ, and is therefore not only the homologue, but also the analogue, of the lungs of the higher Vertebrates. In most fishes the airbladder is an elongated sac with a single cavity, but in many cases it is variously subdivided by septa. In the Mud-fish the air-bladder is composed of two sacs, completely separate from one another, and divided into a number of cellular compartments. The duct (ductus pneumaticus) leading in many fishes from the air-bladder, and opening into the oesophagus, is the homologue of the windpipe (trachea). The air contained in.

the swim-bladder is composed mainly of nitrogen in most fresh-water fishes, but in the sea-fishes it is mainly made up of oxygen. The fishes which live habitually at the bottom of the sea, such as the Flat-fishes, possess no swim-bladder, and it is much reduced in size in those which live principally at the surface.

The nervous system of fishes is of an inferior type of organisation, the brain being of small size, and consisting mainly of ganglia devoted to the special senses. As regards the special senses, there is one peculiarity which deserves particular notice, and this is the conformation of the nasal sacs. The cavity of the nose is usually double, and is lined by an olfac tory membrane, folded so as to form numerous plicæ. Anteriorly, the water is admitted into the nasal sacs by a single or double nostril, usually by two apertures; but posteriorly the nasal sacs are closed, and do not communicate with the pharynx by any aperture. The only exceptions to this statement are to be found in the Myxinoids and in the Lepidosiren. The essential portion of the organ of hearing (labyrinth) is present in almost all fishes, but in none is there any direct communication between the ear and the external medium. In some cases, however, there is a communication between the ear and the swim-bladder, thus foreshadowing the Eusta chian tube in man.

As regards their reproductive system, fishes are, for the most part, truly oviparous, the ovaries being familiarly known as the "roe." The testes of the male are commonly called the "soft roe" or "milt." The products of the reproductive organs are often set free into the peritoneal cavity, ultimately finding their way to the external medium by means of an abdominal pore (or pores); or they are directly conveyed to the exterior by the proper ducts of the reproductive organs.

DIVISIONS OF FISHES.

CHAPTER LIII.

PHARYNGOBRANCHII AND MARSIPOBRANCHII.

THE class Pisces has been very variously subdivided by dif ferent writers; but the classification here adopted is the one proposed by Professor Huxley, who divides the class into the following six orders, in the subdivisions of which Professor Owen has been followed: *

ORDER I. PHARYNGOBRANCHII (= Cirrostomi, Owen; and Leptocardia, Müller).—This order includes but a single fish, the anomalous Amphioxus lanceolatus, or Lancelet (fig. 239), the organisation of which differs in almost all important points. from that of all the other members of the class. The characters of Amphioxus, in fact, are so aberrant, that Hæckel proposes to divide the sub-kingdom Vertebrata into two primary sections the one (Leptocardia) comprising the Lancelet alone, whilst the other (Pachycardia) includes all other Vertebrates. The order is defined by the following characters, which, as will be seen, are mostly negative :-No skull is present, nor lower jaw (mandible), nor limbs. The notochord is persistent; and there are no vertebral centra nor arches. No distinct brain nor auditory organs are present. In place of a distinct heart, pulsating dilatations are developed upon several of the great bloodvessels. The blood is pale. The mouth is in the form of a longitudinal fissure, surrounded by filaments or cirri. The walls of the pharynx are perforated by numerous clefts or fissures, the sides of which are ciliated, the whole exercising a respiratory function.

Cuvier divided the class Pisces into the great orders of the Chondropterygii (or Cartilaginous Fishes), the Acanthopterygii (or Fishes with spinous rays in the paired fins), and Malacopterygii (or Fishes with soft rays in the paired fins). Agassiz divides Fishes, from the character of the scales, into the four orders, Cycloidei, Ctenoidei, Ganoidei, and Placoidei. Müller divides the Fishes into the five orders Leptocardia, (Lancelet), Cyclostomata (Lampreys and Hag-fishes), Teleostei, (Bony Fishes), Ganoidei, (Ganoid Fishes), and Selachia (Sharks and Rays).

The Lancelet is a singular little fish, from one to two inches in length, which is found burrowing in sandbanks, in various seas, but especially in the Mediterranean. The body (fig. 239) is semi-transparent, destitute of an exoskeleton, and lanceolate in shape, and is provided with a narrow membranous border, of the nature of a median fin, which runs along the whole of the dorsal and part of the ventral surface, and expands at the tail to form a lancet-shaped caudal-fin. No true paired fins, representing the anterior and posterior limbs, are present. The mouth is a longitudinal fissure, situated at the front of the head, and destitute of jaws. It is surrounded by a cartilaginous ring, composed of many pieces, which give off prolongations, so as to form a number of ciliated cartilaginous filaments or "cirri" on each side of the mouth. (Hence the name of Cirrostomi, proposed by Professor Owen for the order.) The throat is provided on each side with vascular lamellæ, which are believed by Owen to perform the function of free branchial filaments. The mouth leads into a dilated chamber (fig. 239, b), which is

n

f

a

Fig. 239.-The Lancelet (Amphioxus lanceolatus), enlarged to twice its natural size. o Mouth; Pharyngeal sac; g Stomach; h Diverticulum representing the liver: i Intestine; a Anus: n Notochord; Rudiments of fin-rays; Abdominal pore.

believed to represent the pharynx, and is termed the "pharyn geal" or "branchial sac." It is an elongated chamber, the walls of which are strengthened by numerous cartilaginous filaments, between which is a series of transverse slits or clefts, the whole covered by a richly-ciliated mucous membrane. This branchial dilatation has given rise to the name Branchiostoma, often applied to the Lancelet. Posteriorly the branchial sac opens into an alimentary canal, to which is appended a long and capacious sac or cæcum (), which is believed to represent the liver. The intestinal tube terminates posteriorly by a distinct anus (a), which is situated at the root of the tail a little to the left of the median line; and the intestinal mucous membrane is ciliated. Respiration is effected by the admission of water taken in by the mouth into the branchial sac, having previously passed over the free

branchial filaments before mentioned. The water passes through the slits in the branchial sac, and thus gains access to the abdominal cavity, from which it escapes by means of an aperture with contractile margins situated a little in front of the anus, and called the "abdominal pore" (p). There is no distinct heart, and the circulation is entirely effected by means of rhythmically contractile dilatations which are developed upon several of the great blood-vessels. In other words, the heart retains its primitively tubular condition, and special contractile dilatations are developed upon other vessels (those carrying the blood to the pharynx). The blood itself is colourless. No kidneys have as yet been certainly identified, and there is no lymphatic system. There is no skeleton properly so called. In place of the vertebral column, and constituting the whole endoskeleton, is the semi-gelatinous cellular notochord (n), enclosed in a fibrous sheath, and giving off fibrous arches above and below. The notochord is, further, peculiar in this, that it is prolonged quite to the anterior end of the body, whereas in all other Vertebrates it stops short at the pituitary fossa. There is no cranium, and the spinal cord does not expand anteriorly to form a distinct cerebral mass. The brain, however, may be said to be represented, since the anterior portion of the nervous axis gives off nerves to a pair of rudimentary eyes, and another branch to a ciliated pit, believed to represent an olfactory organ. The generative organs (ovaria and testes) are not furnished with any efferent ducts (oviduct or vas deferens). The generative products, therefore, are shed into the abdominal cavity, and gain the external medium by the "abdominal pore."

ORDER II. MARSIPOBRANCHII (= Cyclostomi, Owen; and Cyclostomata, Müller).-This order includes the Lampreys (Petromyzonida) and the Hag-fishes (Myxinida), and is defined by the following characters:-The body is cylindrical, wormlike, and destitute of limbs. The skull is cartilaginous, without cranial bones, and having no lower jaw (mandible). The notochord is persistent, and there are either no vertebral centra, or but the most rudimentary traces of them. The heart consists of one auricle and one ventricle, but the branchial artery is not furnished with a bulbus arteriosus. The gills are sac-like, and are not ciliated.

The type of piscine organisation displayed in the Marsipobranchii is of a very low grade, as indicated chiefly by the persistent notochord without vertebral centra, the absence of any traces of limbs, the absence of a mandible, and the structure of the gills.