104 



THE POPULAR EDUCATOR. 



COMPARATIVE ANATOMY. XVIIL 



VEETEBEATA. 



THE previous lessons have prepared the student for the con- 

 sideration of tho Vertebrata, the last and greatest division of 

 the animal kingdom. 



In this division, as in others, the Divine Architect has followed 

 out, in all the beauty which his every work evinces, that suc- 

 cessive gradation from a comparatively simple to a more com- 

 plex organisation. Tho Vertebrata are an extensive series of 

 animals, which, though occupying earth, air, and water, and 

 possessing wide differences in their general form, habits, and 

 degree of intelligence, have yet certain characters in common by 

 which the naturalist is enabled to classify them. The step 

 between this and the previous division is not so sudden and 

 complete as to create a distinct line of demarcation. It is by 

 a gradual transition. On the very boundary line of the two 

 divisions there is a little being which forms the connecting link 

 between them, by partaking of the characters of both : this is 

 the lancelet (Amphioxus lanceolatus), so named from its lanceo- 

 late form. It is found in the European seas, especially the 

 Mediterranean. Its respiratory or breathing apparatus is that 

 of an ascidian mollusc ; but it has a rudimentary spine and a 

 spinal marrow, which are decidedly vertebrate. It is this spine 

 or backbone which constitutes the principal feature in the bases 

 of classification. Every animal in possession of a spine, how- 

 soever rudimentary or imperfect, must belong to this great 

 division of Vertebrata. In proportion as the spine is found 

 developed, so will be the other bones which complete the skeleton. 

 Independently of these two characters, the Vertebrata are dis- 

 tinguished by a more highly organised breathing and circu- 

 latory apparatus. They possess a heart, and have red blood ; 

 they have a brain and spinal marrow ; and a corresponding 

 increase in the development of the emanating nerves. They are 

 provided with sensory organs, such as those of hearing, sight, 

 emell, taste, and touch. The anatomy of these several struc- 

 tures will be briefly reviewed under the respective sub-divisions 

 of the Vertebrata. 



This grand division is subdivided into five great classes : 

 Fishes, Amphibia, Reptiles, Birds, and Mammals. 

 FISHES. 



In accordance with the plan previously followed, we must 

 begin our description with that class which presents the lowest 

 organisation namely, fishes. They are the most extensively 

 distributed throughout the giobe, and the most numerous and 

 prolific of the whole division. Wherever water abounds, in the 

 familiar pond, or in the rippling stream of a narrow brook, in 

 lake or river, sea or ocean, there are floating tenants possess- 

 ing an almost infinite variety of shape and size, from the little 

 minnow to the huge shark. Man, the other extreme of the 

 vertebrate kingdom, unable to explore the waters at will, as he 

 beholds the pond or lake whose gentle surface is scarcely ruffled, 

 or the rugged waves of the mighty deep tossed to and fro in 

 mountain masses, can form no conception of the vast numbers 

 of living beings situated beneath, listlessly enduring the one, 

 or revelling delighted in the other. The great Pacific, with the 

 lesser ocean the Atlantic, had been traversed by a living chain 

 ages before adventurous and enterprising man first thought of 

 connecting shore to shore by means of a submarine electric 

 cable ; and the very power which a series of successive dis- 

 coveries made him acquainted with lay buried in some of its 

 depths. 



The student may form a bare idea of their numbers, when in- 

 formed that in the herring fishery off Lowestoft, in 1854, nine- 

 teen millions were caught in that single season. 



It will naturally be surmised that the circumstance of fishes 

 being destined to live in so dense a medium as water, their 

 structure will indicate an especial adaptation to this kind of 

 life. Their bodies present the shape which offers the least 

 resistance to the opposing fluid, being smooth, more or less 

 flattened, or rounded and tapering from the middle towards 

 either extremity. They have no neck, the head joining the 

 trunk immediately. 



\ The body, by means of an air-sac contained in the interior, is 

 rendered nearly of the same specific weight as the fluid in which 

 it is immersed. Forming an ornamental protective covering to 

 the surface of the body are numerous scales attached to folds 

 of the skin, and overlapping each other by their free margins, 



like tiles on the roof of a house. These scales present a variety 

 of shapes in different fishes ; and also of consistence, from a 

 mere membrane to a strong bony plate. Some fishes have no 

 scales. 



Fishes move themselves by means of fins, which serve tho 

 place of limbs in higher animals. The two anterior and pos- 

 terior correspond respectively to the fore and hind legs. The 

 anterior pair are called the pectoral fins. They are invariably 

 situated on the breast, immediately behind the gills. Those 

 situated on the belly are called ventral fins. The single fins are 

 the dorsal (Fig. III., 1, 2), the anal (4), and the tail or caudal fins 

 (7). These fins are supported by filaments of more or lesa 

 power and flexibility. The fins differ in their number and size, 

 and also in the nature of the rays or filaments wh^ch support 

 them. The pectoral or ventral, or both, may be absent; or 

 there may be no fins at all. 



We must now briefly review the principal internal structures 

 in the anatomy of the fish. 



All fishes possess a more or less perfect skeleton, the chief 

 element of which is the spine, or vertebral column. This 

 occupies the axis of the body, and extends its whole length. It 

 is composed, not of one single piece, but of a number of seg- 

 ments, connected together by means of a fibrous material. 

 Each segment is made up of a number of parts. 



The central piece (Fig. V., 1) is named the body. It is shaped 

 like an hour-glass, with the two extremities hollowed out into 

 conical cavities, which sometimes communicate. Several pro- 

 cesses project from the body. Above and below there are two 

 small processes (5, 6), which soon unite together, enclosing 

 arch-shaped spaces ; afterwards continuing onwards as single 

 processes. These are named respectively the upper and 

 lower spinous processes. The upper arch lodges a portion of 

 the spinal marrow; the lower, the large artery of the trunk. 

 Besides these there are two other processes (4, 4), the lateral 

 or transverse, which project from each side of the body. The 

 use of these spines is to give attachment to muscles, and afford 

 them leverage in producing the requisite movements in locomo- 

 tion. In the lowest fishes the vertebral column is represented 

 merely by a continuous mass of a soft consistence, made up of 

 the simplest form of cartilage, running through the whole 

 length of the animal, and known as the dorsal cord. In the 

 lancelet, previously mentioned, the spine consists of a series, 

 from sixty to seventy in number, of very rudimentary segments. 

 The spinal marrow extends the whole length of the column, but 

 there is no trace of a skull or brain. Connected either with 

 these spine segments, or their lateral processes, are a number of 

 smaller processes called ribs. These, when complete, surround 

 the body, enclosing and protecting the soft parts. They unite 

 in front with a chain of bones resembling the breast-bone of 

 higher animals. Sometimes the ribs are wanting. 



There are other little bones which spring from the ribs and 

 vertebrae often very numerous, as in the herring. 



The limits of this paper will not allow a description of the 

 bones of the skull. They are numerous, and present a complex 

 arrangement. 



Teeth. True osseous teeth are found in all the classes of the 

 Vertebrata except birds. The teeth of fishes are generally 

 osseous and plentiful. They present in different fishes a variety 

 of interesting forms. In the perch, they are so slender and 

 minute as to resemble the pile of velvet. In the Chsetodontidae,* 

 a family of bony fishes, the teeth resemble bristles, from 

 whence they derive their name. These fishes are numerous on 

 the rocky shores of warm climates, and are often beautifully 

 and variously coloured. One species of this family, the Chelmon 

 rostratus, an inhabitant of the shores of Asia, possesses the faculty 

 of shooting insects with drops of water projected from the 

 mouth, and seizing them as they fall.f The well-known pike 

 (Esox) has its mouth crowded with innumerable teeth, both 

 sharp and formidable. 



The teeth are attached to the bones surrounding the mouth 

 by means of ligamentous tissue, but not in sockets, like those of 

 the higher Vertebrata. They are frequently movable. There 

 are two rows, generally, in the upper jaw, arranged in a parallel 

 manner. The teeth of the shark are arranged in several rows, 

 the anterior only being perpendicular ; the remainder are recum- 

 bent. (Fig. V.) 



* Kairn, a mane ; oiovt, a tooth. 



t Dr. Baird. 



