CAMBRIAN INVERTEBRATES 



125 



the horseshoe crab {Limulus polyphemns), its nearest modern 

 relative, which is beheved to be not so closely related to the 

 phyllopod crustaceans as would at first appear, but rather to 

 the Arachnida through the eurypterids and scorpions. Mo- 

 laria and Limulus are strikingly similar in their cephalic shield, 



Fig. 23. Theoretic World Environment in Middle Cambrian Time. 



The period of the trilobite Paradoxidcs. This shows the theoretic South Atlantic con- 

 tinent "Gondwana" of Suess, connecting Africa and South America. 



segmentation, and telson; but the latter shows an advance 

 upon the earlier type in the coalescence of the abdominal seg- 

 ments into a single abdominal shield-plate. The trilobate 

 character of the cephalic shield in Molaria is an indication of 

 its trilobite affinities; hence we apparently have good reason 

 to refer both the merostomes and phyllopods to an ancestral 

 trilobite stock. 



Another mode of defense is presented by some of the 

 sessile, rock-clinging sea-cucumbers (Holothuroidea) protected 



