Graptolites

Graptolites, an extinct class of Hydrozoa (q.v.) which lived only in the Cambrian, Ordovician, and Silurian periods. They were all compound animals, and appear to have been always free-swimming. They consisted of two main parts, a numerous series of polypites united by a common flesh or "cosnosarc"; they are represented now by the cbitinous skeleton by which they were protected. This consists of the cuplike "hydrotbecee" in which lived the polypites, and a tubular rod upon which these are placed. At the first-formed or proximal end is a small expansion known as the "sicula," which was the original cell, which either persisted during life as a float, or a vesicle for this purpose is formed from it. The sicula is the earliest known stage in the life history of the graptolites; the next stage is the "virgula," in which a small rod grows up from the sicula; upon this the hydrothecee are formed, and the graptolite stage is reached. A rod sometimes also grows downward from the sicula, and this is known as the radicle. The rod bearing the hydrothecee consists of two parts, a solid cord or axis, and a hollow tube on one side of this: this tube or common canal contained the ccenosarc. The reproductive organs of the graptolites are not certainly known, but there is little tloubt that certain small cuplike organisms, frequently associated with the graptolites, are the "gonophores" which protected the eggs; similar gonophores occur in the Sertularians (q.v.), which are the nearest living allies of the graptolites. These structures were described as Dawsonice, and until they have been found actually united with a graptolite their nature must be regarded as doubtful. The graptolites are classified according to the arrangement of the hydrothecee on the axis; as a rule they overlap, but in one group - e.y. Rastrites - they are loosely scattered. The graptolites with overlapping hydrothecee are divided into the "Monopriodon" and "Dipriodon" groups: in the former the hydrothecee are placed on one side only of the axis, as in Monograptus, Tetragraptus, etc., while in the other they are placed on both sides, as in Diplograptus. In one family they occur on each side of an expanded plate, giving the colony a leaf-like form: such are known as the Phyllograptida.