EVOLUTION OF FOSSIL CEPHALOPODA. 



237 



species is known more accurately than that of most liv- 

 ing forms. 



Vertebrates are out of the question for this sort of 

 work, being too highly accelerated in their development ; 

 the stages that might be useful in phylogenetic study 

 are gone through before the animal is capable of being 

 preserved as a fossil. In the ccelenterates the relations 

 between Cenozoic and Paleozoic forms are not under- 

 stood, and the ontogeny of the group does not show 

 changes that are striking enough to throw much light on 

 their history. In the Echinodermata poor preservation 

 of fossil forms, especially of the young, makes ontogen- 

 etic study very difficult, but Dr. R. T. Jackson * has 

 been able, by a study of development of plates of the 

 sea urchins, to throw some light on the phylogeny of 

 the group. The only crinoid of which the development 

 is known is Antedon, which, unlike all the others, is free- 

 swimming when adult, although attached by a jointed 

 stem as a larva. The investigations of Wyville Thom- 

 son, Bury, and others, show that the embryo is like the 

 larva of certain annelid worms ; after a short free stage 

 this embryo settles down, attaches itself to some object, 

 begins to secrete a stem and jointed calyx, or body cup. 

 In this stage it is like the Ichthyocrinoidea of the Paleo- 

 zoic. Then certain of the plates are resorbed, the char- 

 acter of the jointed stem changed, and the animal is like 

 Pentacrinus of the Mesozoic and Cenozoic. At the end 

 of its larval period Antedon frees itself from its fixed 

 position, loses all resemblance to Pentacrinus, and for 

 the rest of its life is a free-swimming pelagic form. 



The most satisfactory groups for this work are the 

 Brachiopoda, the Mollusca, and the Crustacea. 



* Studies of Palseechinoidea. Bull. Geol. Soc. Amer., vol. vii, 

 pp. 171-254- 



