Fig. 313.—Diodon maculatus.
Fig. 314.—Diodon maculatus, inflated.
C. Molina.—Body compressed, very short; tail extremely short, truncate. Vertical fins confluent. No pelvic bone.
The “Sun-fishes” (Orthagoriscus) are pelagic fishes, found in every part of the oceans within the tropical and temperate zones. The singular shape of their body and the remarkable changes which they undergo with age, have been noticed above (p. 175, Figs. [93], [94]). Their jaws are undivided in the middle, comparatively feeble, but well adapted for masticating their food, which consists of small pelagic Crustaceans. Two species are known. The common Sun-fish, O. mola, which attains to a very large size, measuring seven or eight feet, and weighing as many hundredweights. It has a rough, minutely granulated skin. It frequently approaches the southern coasts of England and the coasts of Ireland, and is seen basking in calm weather on the surface. The second species, O. truncatus, is distinguished by its smooth, tessellated skin, and one of the scarcest fishes in collections. The shortness of the vertebral column of the Sun-fishes, in which the number of caudal vertebra is reduced to seven, the total number being seventeen, and the still more reduced length of the spinal chord have been noticed above (p. 96).
THIRD SUB-CLASS—CYCLOSTOMATA.
Skeleton cartilaginous and notochordal, without ribs and without real jaws. Skull not separate from the vertebral column. No limbs. Gills in the form of fixed sacs, without branchial arches, six or seven in number on each side. One nasal aperture only. Heart without bulbus arteriosus. Mouth anterior, surrounded by a circular or subcircular lip, suctorial. Alimentary canal straight, simple, without coecal appendages, pancreas or spleen. Generative outlet peritoneal. Vertical fins rayed.
The Cyclostomes are most probably a very ancient type. Unfortunately the organs of these creatures are too soft to be preserved, with the exception of the horny denticles with which the mouth of some of them is armed. And, indeed, dental plates, which are very similar to those of Myxine, are not uncommon in certain strata of Devonian and Silurian age (see p. 193). The fishes belonging to this sub-class may be divided into two families—