Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/667

649 MYOLOGY.] ICHTHYOLOGY 649 heterocercal. The caudal fin is suspended from htema- pophyses only, and does not extend to the neural side of the vertebral column. The neural arches &quot;coalesce with the centrum; the interneurals are simple. The abdominal vertebroe have parapophyses, to which the ribs are attached. Only the caudal vertebroe have haemal spines. In the skull of Lepid-osteus the cartilage of the endo- cranium is still more replaced by ossifications than in Poly- pterus, those ossifications, moreover, being represented by a greater number of discrete bones. The membrane-bones in particular are greatly multiplied : the occipital, for instance, consists of three pieces ; the vomer is double, as in Polypterus ; the maxillary consists of a series of pieces firmly united by suture. The symplectic reaches the lower jaw, so that the articulary is provided with a double joint,viz., for the symplectic and quadrate ; the component parts of the lower jaw are as numerous as in reptiles, a dentary, splenial, articulary, angular, supra-angular, and coronary being dis tinct. The sides of the head are covered with numerous bones, and a praaoperculum is developed in front of the gill-cover, which, again, consists of an operculum and sub- operculum. Each hyoid consists of three pieces, of which the middlo is the longest, the upper bearing the largest of the three branchiostegals which Lepidosteus possesses ; a long and large glossohyal is intercalated between the lower ends of the hyoids. There are five branchial arches, the hindmost of which is modified into a lower pharyngeal ; upper pharyngeals are likewise present as in the majority of Teleosteous fishes, but there is no gular plate. Of the ssapulary arch the two halves are separated by a suture in the median line ; the membrane-bones are well developed, only a remnant of the primordial cartilage remaining ; the supraclavicle is very similar to that of Teleosteous fishes, and in a less degree the post-temporal. The base to which the limb is attached is a single osseous plate, supporting on its posterior margin semi-ossified rods in small number, which bear the pectoral rays. The pubic consists of a paired bone. The elements representing a tarsus are quite rudimentary, and reduced in number (two or three). The vertebral column of the Amioidei shows unmistakable characters of the Palaaichthyic type. The arrangement of its component parts is extremely simple. The centra of the amphiccelous vertebrae are well ossified, but the neural ani haemal arches do not coalesce with the centra, from which they are separated by a thin layer of cartilage. All the vertebrae do not possess apophyses : in the caudal portion of Amia only every alternate vertebra is provided with them. The heterocercal condition of the spinous column is well marked ; as in the other Holostei, the hindmost vertebrae are turned upwards, become gradually smaller in size, and lose their neural arches, the haemals remaining developed to the end. Finally, the column terminates in a thin cartilaginous band, which is received between the lateral halves of the fifth or sixth upper caudal ray. The interneurals and interhaemals are simple. Only the abdominal vertebrae have parapophyses, with which the ribs are articulated. The configuration of the skull, and the development and arrangement of its component parts, approach so much the Teleostean type, that perhaps there are greater dif ferences in skulls of truly Teleostean fishes than between the skulls of Amia. and of many Physostomi. Exter nally the cranium is entirely ossified ; the remains of the cartilaginous primordial cranium can only be seen in a section, and are of much less extent than in many Physo- stomous fishes. The immovable intermaxillary, the double vomer, the double articulary cavity of the mandible for junction with the quadrate and symplectic bones, remind us still of similar conditions in the skull of Lepidosteus ; but the mobility and formation of the maxillary, the arrange ment of the gill-covers, the development of a praeoperculum, the suspensorium, the palate, the insertion of a number of branchiostegals on the long middle hyoid piece, the com position of the branchial framework (with upper and lower pharyngeals), are as in the Teleosteous type. A gular plate replaces the urohyal. The scapular arch is composed entirely of the membrane- bones found in the Teleostei, and the two sides are loosely united by ligament. The base to which the limb is attached is cartilaginous ; short semi-ossified rods are arranged along its hinder margin, and bear the pectoral rays. MYOLOGY. In the lowest vertebrate, Branchiostoma, the whole of the muscular mass is arranged in a longitudinal band running along each side of the body ; it is vertically divided into a number of flakes or segments (myocommas) by apo- neurotic septa, which serve as the surfaces of insertion to the muscular fibres. But this muscular band has no con nexion with the notochord except in its foremost portion, where some relation has been formed to the visceral skeleton. A very thin muscular layer covers the abdomen. In the Cyclostomes also the greater portion of the muscular system is without direct relation to the skeleton, and, again, it is only on the skull and visceral skeleton that distinct muscles have been differentiated for special functions. To the development of the skeleton in the more highly organized fishes corresponds a similar development of the muscles : the maxillary and branchial apparatus, the pectoral and ventral fins, the vertical fins and especially the caudal, possess a separate system of muscles. But the most noteworthy is the muscle covering the sides of the trunk and tail (already noticed in Uranckiostoma), which Cuvier described as the &quot;great lateral muscle,&quot; and which, in the higher fishes, is a compound of many smaller segments (myocommas), corresponding in number with the vertebra. Each lateral muscle is divided by a median longitudinal groove into a dorsal and ventral half ; the depression in its middle is filled by an embryonal muscular substance which contains a large quantity of fat and numerous blood-vessels, and therefore differs from ordi nary muscle by its softer consistency, and by its colour, which is reddish or greyish. Superficially the lateral muscle appears crossed by a number of white parallel tendinous zigzag stripes, forming generally three angles, of which the upper and lower point backwards, the middle one for wards. These are the outer edges of the aponeurotic septa between the myocomma?. Each septum is attached to the middle and to the apophyses of a vertebra, and, in the abdominal region, to its rib; frequently the septa receive additional support by the existence of epipleural spines. In connexion with the muscles reference has to be made to the electric organs with which certain fishes are provided. That these have been developed out of muscular substance is more than probable, not only from the examination of peculiar muscular organs (the function of which is still conjectural) occurring in the rays, and in Mormyrus and Gymnarchus, but especially from the re searches into the development of the electric organ of Torpedo. The fishes possessing fully developed electric organs, with the power of accumulating electric force and communicating it in the form of shocks to other animals, are the electric rays (Torpedinidce), the electric sheath-fish of tropical Africa (Malapterwrws), and the electric eel of tropical America (Gymnotus). The structure and arrange ment of the electric organ are very different in these fishes. XII. 82