The Hydrozoa were represented not only by jellyfish but also by the graptolite, which takes its name from a fancied resemblance of some of its forms to a quill pen.  It was a composite animal with a horny framework, the individuals of the colony living in cells strung on one or both sides along a hollow stem, and communicating by means of a common flesh in this central tube.  Some graptolites were straight, and some curved or spiral; some were single stemmed, and others consisted of several radial stems united.  Graptolites occur but rarely in the Upper Cambrian.  In the Ordovician and Silurian they are very plentiful, and at the close of the Silurian they pass out of existence, never to return.

Corals are very rarely found in the Cambrian, and the description of their primitive types is postponed to later chapters treating of periods when they became more numerous.

ECHINODERMS. This subkingdom comprises at present such familiar forms as the crinoid, the starfish, and the sea urchin.  The structure of echinoderms is radiate.  Their integument is hardened with plates or particles of carbonate of lime.

Of the free echinoderms, such as the starfish and the sea urchin, the former has been found in the Cambrian rocks of Europe, but neither have so far been discovered in the strata of this period in North America.  The stemmed and lower division of the echinoderms was represented by a primitive type, the cystoid, so called from its saclike form, A small globular or ovate “calyx” of calcareous plates, with an aperture at the top for the mouth, inclosed the body of the animal, and was attached to the sea bottom by a short flexible stalk consisting of disks of carbonate of lime held together by a central ligament.

ARTHOPODS.  These segmented animals with “jointed feet,” as their name suggests, may be divided in a general way into water breathers and air breathers.  The first-named and lower division comprises the class of the crustacea,—­arthropods protected by a hard exterior skeleton, or “crust,”—­of which crabs, crayfish, and lobsters are familiar examples.  The higher division, that of the air breathers, includes the following classes:  spiders, scorpions, centipedes, and insects.

The trilobite.  The aquatic arthropods, the Crustacea, culminated before the air breathers; and while none of the latter are found in the Cambrian, the former were the dominant life of the time in numbers, in size, and in the variety of their forms.  The leading crustacean type is the trilobite, which takes its name from the three lobes into which its shell is divided longitudinally.  There are also three cross divisions,—­the head shield, the tail shield, and between the two the thorax, consisting of a number of distinct and unconsolidated segments.  The head shield carries a pair of large, crescentic, compound eyes, like those of the insect.  The eye varies greatly in the number of its lenses, ranging from fourteen in some species to fifteen thousand in others.  Figure 268, C, is a restoration of the trilobite, and shows the appendages, which are found preserved only in the rarest cases.