212 



HYDROZOA. 



B 



C 



thecse (fig. 93, b), whereas in the double Graptolites it occupies a 

 central or sub-central position, between the two rows of hydrothecse 

 which form the biserial colony. In any case it lies in a groove in 

 the periderm, and is really outside the coeno- 

 sarc. Very commonly, the virgula projects 

 beyond one or both ends of the polypary as 

 a longer or shorter naked rod (fig. 92, c, and 

 fig. 94, a) ; the proximal extension being 

 often spoken of as the " radicle," though this 

 name has also been used for the " sicula." 

 Running parallel with the virgula is the cceno- 

 sarc — often spoken of as the " common 

 canal" — which is enclosed in the tubular 

 periderm, and presents itself in compressed 

 specimens as a flat space of variable width 

 between the virgula and the bases of the 

 hydrothecse (fig. 94, c). Springing from the 

 ccenosarc, lastly, is a series of chitinous cups 

 or " hydrothecas," which have often been 

 spoken of as the " cellules " or " denticles." 

 Each hydrotheca rests by its base upon 

 the coenosarcal canal, is separated from 

 its neighbours by a definite partition, and 

 opens at its apex by an " aperture," through 

 which the contained polypite could protrude 

 its tentaculate head. The hydrothecse vary 

 extremely in form and in the precise position 

 of the aperture in different types of Grapto- 

 lites, or in different species of the same 

 genus (fig. 94); and the lip of the aperture 

 may be variously ornamented with spines. 

 Lastly, the hydrothecse are typically so dis- 

 posed that they more or less extensively overlap one another (fig. 

 94, b and c). 



While the above is the arrangement of parts in such a form as 

 Monograptus priodon, there are many departures from this typical 

 condition in other forms of the Graptolites. Thus, the general peri- 

 derm, instead of forming a dense continuous membrane, may, as 

 in Retiolites (fig. 106), be reduced to a mere network of chitinous 

 threads. In the same genus, the virgula is double, the two halves 

 being placed on opposite sides of the ccenosarc, and being 

 united with the peridermal network. Again, the hydrothecse, 

 instead of more or less largely overlapping one another, may, as in 

 Rast7'ites (fig. 98), spring quite separately and at greater or smaller 

 intervals from the ccenosarc. Lastly, it has not been certainly estab- 



Fig. 93. — Morphology of 

 Monograptus priodon. a, Mo- 

 nograptus priodon, Bronn, pre- 

 served in relief — lateral view 

 slightly enlarged — from the Si- 

 lurian rocks ; b, Dorsal view of 

 a fragment of the same species 

 — considerably enlarged ; c, 

 Front view of a fragment of the 

 same, showing the mouths of 

 the hydrothecse — much en- 

 larged ; d, Transverse section 

 of the same. (Original.) 



