GRAPTOLITES. 21 



and well mounted. In the center is the animal proper, consisting of 

 an elongated hollow stem, from which at regular intervals arise a pair 

 of opposite flower-like heads ox polyps, connected with the stem by a 

 narrow "neck." The polyps form the swollen end of each tube, 

 which at the summit is contracted into a narrow opening, the mouth, 

 and furnished some distance below the summit with a ring of tenta- 

 cles. The hollow internal space or stomach cavity of each polyp is 

 continuous through the " neck " with that of the main stem, which 

 in turn through the hollow tube of the rootstock is continuous with 

 that of other stems of the colony. At intervals peculiar large 

 polyps without mouth or tentacles occur, which serve the function 

 of reproduction. In some types of hydroids free-swimming medusae 

 or "jelly-fish " are produced by these reproductive or gonopolyps. 

 Enveloping the rootstock, stems and branches, is a transparent 

 membrane composed of the organic substance " chitin," which is 

 similar to the material of which sponges consist. This envelope 

 ox periderm widens around the polyps into cups or Jiydrothecce, into 

 which the polyps can withdraw by muscular contraction, after 

 which the opening is closed by a lid or operculum. In the figure 

 one of the polyps is expanded, the others are contracted or not 

 shown. The gonopolyps are surrounded by a bottle-shaped cup or 

 gonotheca, which has a short neck and broad opening. The sub- 

 stance of the periderm is such that it may be preserved in the form 

 of a carbonaceous film. 



Graptolites. 

 The simplest groups of Graptolites (Axonolipa) , characteristic of 

 the highest Cambric and lower half of the Ordovicic, are derived 

 by a succession of buddings from a primal hydrotheca known as 

 the sicula. If the first two buds extend in opposite directions, the 

 result is a type with branches composed of a single row of hydro- 

 thecse. These may be strung out in long rows, where each suc- 

 ceeding hydrotheca buds from near the margin of the preceding 

 one (Fig. 46), or they may bud rapidly so as to overlap each other. 

 In the former case the serrations are far apart; in the latter, which 

 is the more specialized, they are close together. When the first 

 two buds give off two buds each, four branches result ( Tetra- 

 graptus) instead of two (Didymograptus). If later buds again give 

 off two instead of one bud, a larger number of branches is produced 

 {Staurograptus, Dichograptus ; etc.). When the branches are nu- 

 merous and united by cross -bars or dissepiments a net-like struc- 



