THE MANY-ARMED MOLLUSKS - CLASS CEPHALOPODA. Highly organised marine mollusks, of carnivorous habits, moderate to large size, and of great strength and swiftness of motion. Shell absent, except in Nautilus. Body cylindrical or bag-like, bilaterally symmetrical. Head encircled by arms or tentacles which are furnished with sucking disks or hooks. Mouth fitted with horny beak, and toothed radula. Ink-bag present, except in Nautilus. Funnel expels water from mantle cavity and propels body backward through the water. Res piration by gills. Skin contains pigment spots by which colour of body is changed at will. Reproduction system complex. Sexes separate. Hectocotylised arm in male contains spermato phores. Organs of smell and of hearing present. Eyes complex. Foot modified into tentacles and siphon. Nervous system cen tralised; main ganglia protected by cartilaginous case.
This "head-footed" group of marine animals are so different in their external character from the univalves and bivalves as to raise this serious question : " Is it not all wrong to class them with the mollusks?" With the exception of the Nautilus no mod ern cephalopod has an external shell such as snails and clams have. Instead, there is in a large proportion of the group an internal shell, familiar to us in the chalky "cuttle bone" of Sepia and the transparent "pen" of the squid. The complicated eye, the highly organized nervous and reproductive systems, also suggest relationship with vertebrates.
The predatory life led by these mollusks, their remarkable strength, agility and skill in running down their prey and in escap ing enemies, the wonderful mechanism of the funnel, the button and-button-hole system that opens and closes the mantle chamber in certain genera, the sucking disks, retractile hooks, and cushions on the arms of others, all give proof of high specialisation.
But the alimentary system, the beak and rasping, toothed tongue, the siphon, and mantle and gills are all molluscan char 439 The Many-armed Mollusks acters. So are the circulatory and excretory systems. Though isolated far from the other members, they are mollusks.
Geologists have found a most thrilling history of the gigantic prehistoric cephalopods written in the rocks. Regarding the forms which had no hard parts, much is left to conjecture, but the shell-bearing cephalopods are in evidence, remarkable in variety, size and numbers, from the Upper Cambrian rocks, through the Silurian, Devonian and Carboniferous eras, cul minating in the Mesozoic time, and dwindling in the Cenozoic.
Silurian rocks yield fossil "straight-horns" fifteen feet long. The gigantic Ammonites, with coiled and chambered shells four feet in diameter, mark the maximum point of development reached by the ancient tribes of Cephalopods. In the eras fol lowing the Cretaceous, which saw this wonderful tribe decline, the mollusks are distinctly modern in size and genera.
A brief account of these fossil cephalopods will be found in any text-book of geology.
Cephalopod mollusks are divided into tWo orders, based upon the number of gills and presence or absence of an external shell. Subdivision into sub-orders is based upon the number of arms that surround the head. A simple key will set forth these groups.