This very small class of animals is distinguished by the leathery sac-like covering, from which they take their name of Tunicata (having a tunic). The Ascidian[28] is the best known representative. It is found fastened to rocks, shells, crabs, and other bodies. These animals are both simple and compound; the latter are often phosphorescent. They have neither feet, head nor shell, but a shapeless body with apertures at both ends.
This division includes the most perfect animals. Their chief distinguishing characteristics are an internal skeleton; a backbone; a dorsal nervous cord, separated from the body cavity; a complete circulation, and limbs not exceeding four. There are about 25,000 living species, beside the numberless host now extinct.
Lowest of the vertebrates, and closely related to the true fishes, are the Lancelet and Lamprey.[29] The former is a lance-shaped animal having no skeleton, but boasting the rudiment of a backbone in a string-like cartilaginous cord. The organs are very simple, the heart being a long sac in which colorless blood circulates. It breathes by taking in water through the mouth and letting it out through the gill-slits. The Lamprey belongs to the pouch-gilled vertebrates or _Marsipobranchii_.[30] It is an eel-shaped animal of about three feet in length.
The round, soft mouth is suctorial, the tongue acting as a piston. By suction it can anchor itself to a rock, and allow the long body to float freely, without being carried by the current from the place. There are seven gill openings on each side, and the whole breathing apparatus so arranged that the animal can live some days out of the water. In some parts of England it is in demand for the table, and “lamprey pie”[31] is esteemed a great luxury. The American lamprey is similar, and the flesh good, but in less demand in our markets. During the breeding season those about the estuaries go up stream as do the shad, and by rolling stones together construct large conical nests[32] for the protection of their young. With apparently little adaptation for such architecture they accomplish wonders in that line.
CLASS I.—_Pisces._ The first clearly defined division of vertebrates is that of the fishes. They are regarded as, in some respects, the lowest of vertebrate animals. They are credited with having the least intelligence and sensibility. Their eyes, though often large, are nearly motionless in their sockets, are protected by no eye-lids, and are without the expression usual even in the animal eye. If they have ears the external parts are wanting. Sounds may reach the auditory nerve and be heard through the cranium. The other senses, as taste, touch and smell, are but slightly developed. But they are admirably adapted to the element in which they live, and the mode of life for which they were created. In no other organisms is the evidence of design in the adaptation of means to the ends contemplated, more apparent. In the number of species and variety of forms they exceed all other vertebrate animals.
