EVOLUTION IN THE PAST 



WORMS 



LAMPSHELLS 



BIVALVED 

 MOLLUSCS 



UNIVALVED 

 MOLLUSCS 



CEPHALOPODS 



Worms, no doubt, were abundant ; but there is little 

 evidence of their existence beyond the marks of their trails 

 and burrowings. 



Among the most flourishing animals were the brachiopods 

 or lampshells. These creatures were enclosed in two-valved 

 shells, more or less lamp-like in outline. Some of them lived 

 almost buried in the sand (Lingula), and their line has lasted 

 till now. The majority, no doubt, passed their lives chnging 

 to rocks by means of a muscular stalk protruded through the 

 shell- valves. Brachiopods may have been the first animals to 

 possess the semblance of a heart. At some period in their 

 career their heads — except the mouth — certainly disappeared. 

 In a rock-clinging existence, an improved circulation, it 

 may be supposed, was a greater boon than a head-piece. 



The shells of the brachiopods were in different stages of 

 evolution. In the simplest forms the valves were not fur- 

 nished with a hinge {Paterina, Oholella). Neither was there 

 a special opening for the protrusion of the stalk ; and for its 

 outlet, therefore, the valves must always have been gaped to 

 a certain extent. Other brachiopods were more securely 

 ensconced, as there was a special aperture through which the 

 stalk was protruded (Discinolepis, Kutorgina). The shell- 

 valves, moreover, of these animals could be opened and closed 

 with far greater precision owing to the presence of a hinge, 

 regulated by teeth and sockets. This mechanical contrivance 

 — the earUest known piece of machinery in Nature — enabled 

 the animals to offer a very firm resistance to would-be in- 

 truders. 



The rocks were also the abiding-places of some forms of 

 bivalved molluscs — primitive mussels and arksheUs (Modio- 

 loides, Glyptarca). Here also were hard-chnging univalves, 

 shelled like modern limpets (Scenella). Other gastropods, 

 some in ramshorn shells, some in shells of periwinkle shape, 

 crawled about the shallows (Raphistoma, Holopea). Free- 

 swimming forms glided through the waters, but their shells 

 were not of the pattern of any " winged " snails now hving 

 (HyoUthes). Cephalopods related to the Nautilus were also 

 on the scene. Their shells, however, in being either straight 

 or but slightly curved, were very different from the tightly 



