42 HYDROZOA. GRAPTOLITOIDEA. 



frequently circular, but sometimes quadrangular or slit- 

 like. On the dorsal side of the polypary, that is, opposite 

 the hydrothecae, there is on the outer surface of the 

 periderm a longitudinal groove (fig. 8 c), in which is 

 placed a chitinous thread or rod, termed the axis or 

 virgula (fig. 8 d, a) ; in some cases this is solid, in others it 

 appears to be hollow. In some species of Monograptus 

 the virgula projects beyond the distal end of the common 

 canal. At the proximal end of the polypary there is a 

 small elongated cone-like body, of an embryonic nature, 

 termed the sicula (fig. 8 e, s). 



With regard to the soft parts of the animal, speaking 

 from analogy with recent allied forms, we may suppose, 

 that each hydrotheca lodged an individual polypite, and 

 these were connected by means of the ccenosarc which 

 occupied the common canal. 



In the form just described (Monograptus) the polypary 

 is always simple, but in many genera it is compound, 

 consisting of two or more branches. When the branches 

 are given off from a common centre, their proximal 

 parts are sometimes enclosed in a horny sheath termed 

 the central disc 1 , as in some species of Tetragraptus. 

 In some cases (e.g. Monograptus, fig. 8 e) the polypary 

 possesses only a single row of hydrothecae, such forms are 

 said to be monoprionidian ; others however (e.g. Diplo- 

 graptus, fig. 8 g) possess two rows on opposite sides of 

 the polypary, these are the diprionidian forms, and they 

 may have a single common canal (e.g. Retiolites), but 

 more usually there are two (e.g. Diplograptus), and when 

 this is the case, the polypary appears to consist of two 

 monoprionidian forms united by their dorsal surfaces. In 



1 In some simple forms (e.g. Climacograptus bicornis) there is fre- 

 quently a similar disc at the proximal end. 



