﻿150 T. Davidson — What is a BracMopod? 



difficult a subject. I am, however, truly happy to state that this 

 important inquiry has been most ably and successfully elaborated 

 during the last forty years by some of the most distinguished anato- 

 mists and naturalists of the period. To such men as Hancock,^ 

 Cuvier,^ Owen,' Huxley,* Gratiolet,^ Vogt,^ Macdonald,'' King, and 

 others, we are indebted for an extensive series of dissections and obser- 

 vations which have defined, to a very considerable extent, what are the 

 true characters of the Brachiopod ; while some important researches 

 elaborated by Steenstrup, Lacaze-Duthiers, Morse, Dr. Fritz Muller, 

 Oscar Schmidt. McCrady, Kowalevsky, and others, have thrown much 

 additional light upon the embryology and early stages of the group. 

 Some differences in opinion, it is true, have been and are still enter- 

 tained with respect to the exact function to be attributed to certain 

 parts of the animal ; but on all essential questions there is a pretty 

 general agreement. 



Before describing the various parts of the animal, it may be as well 

 to mention that the BrachioiDods have been divided by Bronn into 

 two great groups, termed Apygia and Fleuropygia. Professor King, 

 considering these to be inadmissible on certain grounds, substituted 

 the name Clistenterata for the first group, on account of its including 

 animals that are destitute of an anal aperture ; and the term Tret.en- 

 terata for the second, as it embraces animals provided with this 

 opening. The former division contains species which have the valves 

 articulated, as Terehratula, Spirifer, Rhynclionella, etc. The latter 

 comprise species with non-articulated valves, as Lingula, Discina, 

 etc.** Some very important modifications in the animal are con- 

 nected with these two divisions, especially in what relates to the 

 muscular system. 



According to Morse the Brachiopods are reproduced by eggs, 

 generally kidney-shaped and irregular, which are discharged from 

 the anterior margin of the shell, and drop just beyond the pallial 

 membrane, hanging in clusters from the setee. Some uncertainty 

 has prevailed as to whether there is a male and female individual. 

 Lacaze-Duthiers and Morse state that the sexes are separate, and 

 describe them as such in Thecidium and Terebratulina, and the 

 French malacologist goes so far as to suggest that a difference is 

 even observable in the shell ; but this point I am unable to deter- 

 mine. 



Prof. Morse describes the embryo of Terehrakdina with great 

 minuteness during its six stages of development. It is divided into 



1 Philosophical Transactions Eoyal Society, vol. 148, 1858. 



- Sur I'animal des Lingules, BiUl. Soc. Philomatique de Paris, vol. i., 1797, and 

 Sur I'animal de la Lingule anatina, Memoirs du Museum, vol. i., 1802. 



•5 Transactions of Zool. Soc, vol. i., 1833, and Davidson's General Introduction 

 to British Fossil Brachiopoda, vol. i., 1853. 



^ Annals and Mag of Nat. History, vol. xiv., 2nd series, 1854. 



5 Journal de Conchyliologie, 1857, 1859, and 1860. 



^ Anatomie der Lingula anatina, 1845. 



■^ On the Physiology of the Pallial sinuses of the Brachiopoda, Transactions of the 

 Linnasan Society of London, vol. xxiii., p. 373, 1862. 



^ An instructive note on the primary divisions of the Brachiopods by T. Gill will 

 be found in the Annals and Mag. of Natural History, 4th, series, vol. xii., 1873. 



