218 



ICHTHYOLOGY. 



frequentiy they have strange irregular forms (fig. 44), and 



of acutely bent dermal bones, and the Dory has two rows Introduc- 

 of plates along the belly. In the OstracionidcB and Sijn- tion. 



Dermal 



Fig.«. 



Fig. 43. 



Fig. 43. Figured Cycloid scale from tlie lateral line of Ectmeroco'tes acantho- 

 rhynchus, a Gobioid inhabiting tlic seas of New Zealand. 

 ... 44. Scale from the lateral line of Trifjla pleuracanthica, one of the 

 Australian ScUroffenidw, 



they are in some cases peculiarly armed. The lateral 

 line may be traced forwards on the head to one or more 

 little bones named supra-temporals, and thence over the 

 cheek along the sub-orbital series of bones to the first and 

 usually largest of the series, named the pre-orbital, as may 

 be well observed in fig. 8, page 207. All these bones are 

 constructed so as to protect the muciferous canals, and 

 in many fishes they are mere osseous pipes. In the 

 GadidcB and in Macrotirus, the pre-orbital and following 

 bones of the chain are largely developed and fold down, 

 forming lar^e serai-canals. These bones are strongly armed 

 in some Acanthopteri, particularly the SclcrogmidcE, in 

 which the second of the sub-orbitar chain emits a spine or 

 series of spines or tubercles, and is prolonged to articulate 

 with the pre-opercular. The marginal serratures of the 

 pre-orbital are conspicuous in Mugil, and fiirnish charac- 

 ters for other genera of Acanthopteri. The supra- 

 orbitals, though less constant than the sub-orbitals, 

 belong to the same system. The Carp has one above 

 each orbit, and the Lepidosteus three. The turbitials are 

 also considered by Professor Owen to appertain to the der- 

 mal system. They are wanting, together with the sub- 

 orbitals, in many of the serpentiform apodals ; but the tur- 

 binals exist, though of very small size, in the Conger. 



Of the other dermal bones of fishes our notice must be 

 very cursory. The head of the Sturgeon is protected by 

 large dermal plates closely arranged, and the rows continued 

 along the trunk of the fish are, as we have mentioned above, 

 examples of the Placoid scales of Agassiz. The sternum, 



gnatliidtB the fish is wholly or completely encased in scales, 

 so connected as to admit of few motions of the body. And 

 in the Gurnards, and many other Sclerogenida, some 

 genera of Silurida, the great Sudis among the Clupeidce, 

 and Pohjpterxis among the Ganoids, the cranial and hu- 

 meral bones of the endo-skeleton present hard, rough, sha- 

 greened surfaces exteriorly, in which the soft epidermis is not 

 perceptible, and the resemblance to the dermal plates of the 

 Sturgeon is close. In many other fishes, also, the cranial 

 bones emit processes from their outer surfaces, which serve 

 as canals for the mucoducts. 



SECTION IT. — THE MUSCLES AND MUSCtTLAR MOTIONS OF 

 FISHES. 



The spinal column, composed of numerous articulations 

 united by cartilages which permit of certain movements, 

 curves with great facility from side to side ; but the vertical 

 motion is much more restricted, chiefly in consequence of 

 the projection of the upper and under spinous processes of 

 the vertebras. 



The great organ of movement in all fishes is the tail. 

 The muscles by which it is brought into play extend in 

 lengthened masses on either side of the vertebral column. 

 The body being rendered specifically lighter by the dis- 

 tention of the swim-bladder when that exists, is impelled 

 forwards by the rapid flexure of the tail acting laterally 

 upon the water. The head of fishes exercises but slight 

 movement, independent of the rest of the body ; but the 

 tympano-mandibular arch, often the maxillary and pre- 

 maxillaries, the hyoid and branchial apparatus, and occa- 

 sionally the scapular arch, have more or less motion. The 

 finny terminations of the last named appendage and of the 

 pelvic bones, as well as the vertical fins, are seldom at rest. 



Skeleton. 



Fig. 45. 



Head of BovicMhy$ variegnhis, one of the Uranoscoijidtc from Sydney, to show 

 the branching mucoducts. 



as it has been called, of the Herring, is formed of 



SECTION V. — THE NERVOUS SYSTEM AND SENSES OF FISHES. 



The spinal chord, which terminates anteriorly in an en- 

 largement named the brain, consists of a dorsal and ventral 

 pair of columns, the former having for its function the re- 

 ceiving of impressions made on the peripheral surfaces of 

 the body by external objects, and the latter, that of direct- 

 ing the voluntary muscular movements, — they are the 

 nervous centres of animal life. Each consists of an inti- 

 mate aggregation of nervine matter named ganglions. 

 From the sensorial and motor columns on each side of the 

 chord, nervovis filaments, more or less in number, proceed, 

 and being invested by a common membrane, pass out be- 

 tween the vertebra; to all parts of the body ; one on each 

 side between every pair of vertebras, the number of verte- 

 brae being in fact regulated primarily by the number of 

 pairs of spinal nerves. It would be correct to say that the 

 nerves of motion arise from the motor column, and that 

 the nerves of sensation terminate in the sensorial one. For, 

 though the trunks of the nerves lie in a common envelope, 

 the distribution of the former may be com])ared to a tree 

 having one or more roots, with a tnmk giving origin to 

 numerous branches, and innumerable twigs ; while the sen- 

 sorial nerves are like streams formed by minute rills, 

 uniting in a river that terminates by one or by several mouths. 

 To serve the involuntary muscles, as the heart and arterial 

 system, and also to supply the viscera that perform the 

 functions of digestion, assimilation, secretion, or to preside 

 over organic life there is a separate system of ganglia and 

 network of nerves that has been named the sympathetic. 

 Under this general description the nervous systems of all 

 the vertebrals may be comprehended ; but in the higher 

 a series classes, the development is much more full than in fishes, 



