126 COELENTERATA— HYDROZOA phylum ii 



smooth or fiiiely striated chitine ; usually it has the form of a dense continuous 

 membrane, but in the Retiolitidae it is attenuated and supported by a latticed 

 network of chitinous threads. It is usually preserved as a thin bitumino- 

 carbonaceous film, which, however, is often infiltrated with pyrites, and is not 

 infrequently replaced by a glistening greenish-white Silicate (Gümbelite). 



The Compound organism or rhabdosome ('^ polypary ") of the Graptolites is 

 usually linear, more rarely petaloid in form, undivided or branching, and is 

 either straight, beut, or in exceptional instances spirally enrolled. These 

 rhabdosomes, each of which originates from a sicula (see below) may again be 

 united into colonies of a higher order (synrhabdosome). Cup-shaped rhabdo- 

 thecae, which are usually obliquely set and more or less overlapping, are borne 

 on one or on both sides of the polypary, and are united by a common 

 coenosarcal canal enclosed in the /periderm. The polypary is in later forms 

 strengthened by a peculiar chitinous axis (virgula, solid axis), which in the 

 Monograptidae runs in a groove lying outside the coenosark on the dorsal side 

 of the organism {i.e. on the side opposite to the theciferous margin). But in 

 the biserial Graptolites the virgula is either enclosed between the laminae of 

 a central or sub-central septum, which is formed by the coalescence of the 

 flattened dorsal walls (Biprionidae) ; or it is double and the two virgula (see 

 text Fig. 209) are placed on opposite sides of the coenosark, and are united with 

 the peridermal network {Retiolitidae). 



Springing from the common canal, is a series of thecae {celhles, denticles), which 

 are disposed in longitudinal rows along either one (Fig. 193), two (Fig. 194) 

 or four sides of the polypary. They usually have the form of elongated, 

 cylindrical, rectangular or conical sacs; their walls are in most cases applied 

 to those of their neighbours above and below, although occasionally they 

 spring out quite isolated from one another. Each theca opens directly into 

 the common canal, and is furnished distally with an external aperture, the 

 form and size of which vary extremely in difFerent species. In some forms it 

 is circular or quadrate or introverted or introtorted ; in others it is contracted. 

 Not infrequently the outer lip is ornamented with one or two slender spines, 

 which often subdivide and inosculate with one another. The form of the 

 thecae and apertures has been employed by Lapworth to define families and 

 subfamilies. 



The polypary in most Graptolites is furnished at its proximal end with a 

 minute, triangulär or dagger-shaped, originally conical, body called the sicula 

 (Fig. 195), which represents the original embryonic skeleton and is suspended 

 from an originally tubulär filament, the 7iema oy nemacaulus {Fig. 196). In the 

 wall of the sicula is formed, in the later Graptolites, an axis or rod, the 

 virgula, which extends through the rhabdosome. Rhabdothecae are then 

 budded either uniserially along one side, or in alternate sequence along both of 

 the lateral margins of the sicula, originating from one theca near the major end 

 of the sicula. They grow either laterally away from the sicula (Axonolipa) or 

 along the nemacaulus (Axonophora). The sicula itself ceases to grow, as a 

 rule, after the first thecae are budded, and sometimes it becomes obsolete or 

 absorbed. Sometim^es the rhabdosome remains undivided, sometimes it forms 

 branches, which may diverge at various angles ; in other cases two or four 

 uniserial polyparies may be placed back to back with their dorsal walls 

 coalescing, thus giving rise to di- or tetra-serial colonies. In the latter types 

 the coenosarc is commonly divided by one or two median septa. 



