150 THE INVERTEBRATA 



were independent planktonic organisms with a pneumatocyst or other 

 kind of float. Such a pneumatocyst appears to be shown in Fig. 126 of 

 Diplograptus as a square central body from which a number of stipes 

 radiate. There is also a circle of round bodies which are possibly 

 gonophores, as they contain siculae. In any case the graptolites were 

 true pelagic coelenterates and their floating habit gave them a uni- 

 versal distribution in the Palaeozoic oceans. A series of life zones may 

 be traced in the rocks which were there laid down, each characterized 

 by a definite assemblage of graptolites, and these may be traced 

 throughout the world. By a careful consideration of these graptolite 

 successions the main line of evolution of the group has been worked 

 out. It is now concluded that actual genetic relationship is best traced 

 by the characters of the hydrothecae. The earlier forms have very 

 simple hydrothecae, but the shape becomes gradually more complex. 

 On the other hand the genera were usually founded on the number of 

 branches or stipes in the colony, such as Bryograptus with many stipes 

 in the Cambrian, Tetragraptus with four in the Lower Ordovician, 

 and Didymograptus with two in Lower and Middle Ordovician. 

 These genera succeed each other in geological age, and so we may 

 suppose that they constitute an evolutionary series. In reality they 

 constitute not one but several series. Thus there is the same type of 

 hydrotheca (which we will call A) in Bryograptus callavei, Tetra- 

 graptus hicksi and Didymograptus affinis, while another type (B) is 

 common to B. retroflexus, T. denticulatus and D. fasciculatus . The 

 genera of graptolites as at present constituted are thus open to criti- 

 cism ; it would be more correct to classify all the species into hydro- 

 thecae of type A as one genus, and those into type B as another. In 

 the genus Monograptus, which is the last and most abundant of the 

 graptolites, though the form of the colony is simple, the hydrothecae 

 vary tremendously, and it is obvious that we have here grouped to- 

 gether the descendants of many diflPerent genera. 



Certain forms, whose relationship is not clear, occur very com- 

 monly in the Cambrian and are grouped together as "dendroid" 

 graptolites. It is possible that they are closely related to the Calypto- 

 blastea. 



Class SCYPHOMEDUSAE (SCYPHOZOA) 



This class contains the common jellyfishes of temperate and colder 

 seas, some of which are of extraordinary size, like Cyanea arctica, 

 the diameter of whose disc is a couple of yards. 



The simplest type of Scyphomedusae is found in the division 

 known as the Stauromedusae, two members of which, Haliclystus and 

 Lucernaria (Fig. 127), are not uncommon on the British coasts, 

 adhering to the blades of Zoster a or Laminaria. It has a narrow 



