LIFE IN PRIMARY PERIOD 207 



" bracket " (Didymograptus), in three (Cyrtograptus), or four 

 (Tetragraptus), or in a large number of radiating branches 

 (Dichograptus), curved in the shape of an " S " and bearing 

 branches along its whole length (Ccenogr aphis), twisted in the 

 form of a helix (Monograptus turriciilatus), or, finally, arranged 

 in the form of a net (Dictyonema). The general view is that 

 these are all ttydroid polyps. Allmann, however, pointed 

 out that the opening of the chambers was too small to allow 

 a polyp with tentacles to pass through, and he also called 

 attention to the fact that in the Plumularice there are two 

 kinds of chambers, one large, inhabited by polyps, the other 

 smaller, sheltering only fishing filaments, the dactylomerids, 

 which can emit protoplasmic filaments from their surface, 

 capable of capturing prey and digesting them, and con- 

 sequently of feeding themselves. He noticed also that at 

 first the colonies of the Plumularias were formed exclusively of 

 these small chambers with their fishing filaments, and he was 

 therefore fully justified in considering the Graptolites as 

 Hydroids which had persisted in this state although adult. An 

 important discovery made by R. Ruedemann, however, has 

 altered the whole question. It was formerly believed that the 

 stems so abundantly found in the Silurian schists were free, 

 and they were classified in as many different species as forms 

 discovered. In reality stems presenting very different 

 characters were attached to a soft body formed generally of 

 a kind of central globe surrounded by a crown of smaller 

 globes between or under which the stems with their chambers 

 were attached. This body has been regarded as a mere floating 

 organ, but this would greatly diminish the importance of so 

 large an organ, which would seem moreover to have been 

 provided with a mouth. It seems more likely that this was 

 the real organism comparable in structure to a Medusa or 

 Cydippe, which both have long tentacles, and that the stems 

 with their chambers are really, as Allmann thought, fishing 

 filaments, nematophores or dactyomerids invested with a rigid 

 coating instead of being free and flexible. How were the 

 helicoidal Graptolites, whose axes are connected by an inter- 

 mediary network, attached to the central organism ? That 

 question has not been answered. 



The study of existing hydroid polyps is very instructive. We 

 have already seen how they gave rise to the large bell-shaped 



