CAMBRIAN PERIOD 19 



coiled encasements of their now living relations (Orthoceras, 

 Cyrtoceras}. 



Exceedingly numerous and widespread were trilobites TRILOBITES 

 crusted animals with the body-covering segmented, and 

 thrice-ridged from head to tail. They lived more or less in 

 large companies in muddy waters, and were apparently the 

 first animals to live gregariously. They varied greatly in 

 size ; the smallest being but as grains of sand ; whilst the 

 biggest were broad-shouldered, and measured as much as 

 eighteen inches in length (Olenellus, Paradoxides\ Some were 

 endowed with eyesight ; but many, having taken to a life 

 beneath the mud, had become blind. A few forms appear 

 to have been able to roll themselves up in woodlice fashion. 



Small kindred forms were well shielded on the anterior PRIMITIVE 

 part of the body, and possessed spike-like tails (Aglaspis). KING-CRABS 

 In development they appear to have been intermediate 

 between trilobites and king-crabs, and gave promise, there- 

 fore, of a new group of animals. Some small scorpion-like 

 forms (Eurypterids) were also in the waters (Redlichia, SEA- 

 Strabops'). SCORPIONS 



Species of ostracods were to be seen (Primitia, Entomidella), OSTRACODS 

 and primitive barnacles. Small phyllopods were also abroad, BARNACLES 

 resembling the modern Apus a relation of water-fleas. PHYLLOPODS 



Yet other crustaceans were long-tailed and shrimp-like in SHRIMP-LIKE 

 appearance, with the fore part of the body enclosed in a bent FORMS 

 hood-like carapace (Hymenocaris). These little creatures had, 

 no doubt, descended from the same stock as the phyllopods ; 

 and they were probably ancestral forms of the higher crusta- 

 ceans of later times. Of animals now living the small shrimp- 

 like Nebalia retains the closest affinities with them, and may, 

 therefore, be regarded as one of the " old blood " of marine 

 society. Some allied forms had pod-shaped carapaces ; and 

 have, therefore, been termed pod-shrimps (Caryocaris). 



Marine invertebrate life was, therefore, well represented 

 in the Cambrian ; but, in spite of a never-ceasing Evolution, 

 the times were not exciting. Animals for the most part 

 lived either buried in the mud, or rooted to the ground, 

 or clinging in close-drawn shells to briny rocks. Such as 

 indulged in locomotion moved about in most cases very slowly. 



