JURASSIC PERIOD 83 



Although they had lost their stems, they held fast to the 

 sea-floor by means of a small outgrowth at the base of the 

 cup (Cotylederma). Crinoids of this character are still living 

 in the Caribbean Sea (Holopus). 



Many of the Jurassic starfishes seem to have been in- STARFISHES 

 distinguishable from species now living (Asterias, Astropecten, 

 Luidia). Some new forms were appearing in which the 

 central disc was being extended into the spaces between the 

 arms ; and the animals presented a five-sided cushion 

 appearance (Goniaster}. These, no doubt, were ancestors of 

 the so-called " cushion-stars " of our own day. Some other 

 forms had exceeded the numeral characteristic of the Order 

 and indeed of all " spiny-skinned " animals by develop- 

 ing more than five arms (Solaster). Starfishes of this 

 heretical description have continued until the present day, 

 and are represented by " sun-stars " of many genera. 



Brittle-stars had undergone some trivial modifications ; BRITTLE- 

 and certain of the creatures were apparently identical in STARS 

 structure with brittle-stars now living (Ophioglypha). 



Although " sea-lilies " were reviving, and starfishes and SEA-URCHINS 

 brittle-stars were assuming modern aspect, sea-urchins con- 

 tinued predominant among the " spiny-skinned " inverte- 

 brates. 



The most notable feature of Jurassic sea-urchin life was 

 the appearance of many forms no longer characterised by 

 globular tests. Numbers were now in existence in flattened 

 and four-sided body-coverings (Clypeus, Echinobrissus) ; and 

 the mouth was being brought forward from its old central posi- 

 tion on the under-part of the test. These modifications of the 

 test point, no doubt, to changes of habit on the part of the 

 animals. In locomotion, for instance, the test would neces- 

 sarily become exposed to special resistance from the sur- 

 rounding medium, and tend, therefore, to lose its globe- 

 shaped symmetry. This would markedly be the case where 

 boring and crevice-haunting habits were being adopted. 

 And there can be little doubt that many Jurassic sea-urchins 

 spent less sedentary, open-water lives than their ancestors, 

 and also indeed than most of their contemporary brethren. 



A few new forms, reviving an old fashion, were now 



