214 



RAMBLES AND REVERIES. 



similarity of form to that of the typical trilobite 

 of the Cambrian age. The principal trilobites of 

 this geological period were characterised in general 

 by having the thorax divided into a larger number 

 of segments than is the case with those of later 

 ages, while the tail portion is not so well developed, 

 and the side lobes are profusely fringed, as may be 

 seen in Paradoxides (Fig. 51). The Serolis, which 

 is a crustacean isopod plentifully occurring in the 

 seas around Tierra del 

 Fuego, possesses all these 

 features, and, in addition, 

 has compound sessile eyes 

 arranged in crescentic lobes, 

 just as many trilobites 

 have, while it also resembles 

 the Silurian Phacops 

 caudatus (Fig. 16) in hav- 

 ing a movable tail-shield. 

 There are differences of 

 structure in regard to the 

 mouth organs and antennae, but these are fragile 

 appendages, and would not easily be preserved as 

 fossils, even if trilobites possessed them, which it is 

 thought was the case with some species. 



The Apus productusj a common denizen of our 

 ponds, has also been suggested as being related to 

 some species of trilobite. The Apus in passing 

 through the Nauplius stage takes on a trilobite 

 appearance, and when it is fully developed, its sixty 

 pairs of legs cannot but remind us of the marginal 

 fringes which adorn so many of the older trilobites 



FIG. 63. Scrolls Fabricii, 

 with eye magnified. 



