DEVELOPMENT OF THE BRACHIOPODA 247 



Something is known, therefore, of the early stages in each 

 of the four groups or orders proposed by the writer. 2 The 

 Atremata, Neotremata, arid Protremata are represented by a 

 single genus only in each; Grlottidia, Discinisca, and Laca- 

 zella, respectively; and the Telotremata, by Cistella, Tere- 

 bratulina, and Liottiyrina. Were Grlottidia and Discinisca as 

 well known as Cistella, Terebratulina, and Lacazella, some 

 comparisons could undoubtedly be made which would en- 

 lighten many obscure points of anatomy and morphology, as 

 well as give clearer insight into the history and origin of 

 each group. 



Cistella and Terebratulina are taken as standards of the 

 embryological development on account of the completeness 

 with which they have been studied, and because their points 

 of difference are not great. Lacazella shows such peculiar 

 features that its history must be discussed separately. The 

 nepionic Grlottidia and Discinisca, too, present characters 

 which evidently had an early history somewhat different 

 from Cistella or Terebratulina. * 



In taking up the review of the observed stages of growth, 

 an attempt will be made to fix their limitations. To this 

 end the admirable nomenclature proposed by Hyatt 9> 10 is 

 here adopted, as it is more convenient and of wider applica- 

 tion and significance than the terms heretofore used. Thus 

 far this system has been employed principally in studies relat- 

 ing to the Mollusca, and its application to the Brachiopoda 

 will necessarily require some illustration and explanation. 

 In the preface to " Genesis of the Arietidse " Hyatt has pre- 

 sented a summary of the theoretical opinions resulting mainly 

 from his studies in the Cephalopoda. It is believed that 

 nearly the same ground may be covered in the Brachiopoda, 

 and thus the truth of these deductions will receive further 

 evidence from another class of organisms. 



Embryonic Stages. 



The true embryonic stages are classified by Hyatt as Pro- 

 tembryo, Mesembryo, Metembryo, Neoembryo, and Typembryo* 



