OF CONCHOLOGY. 49 



species) do not indicate a strict metamorphosis, such as would be 

 inferred from Miiller's figure, and apparently exists in Theci- 

 dium, as shown by the researches of Lacaze Duthiers. It is 

 probable that the embryonic stages differ widely in different 

 forms of Brachiopods, as they do in the Ascidians and other 

 groups ; and no reliance can be placed upon them in classifica- 

 tion, as far as can be judged at present. 



The position of the valves with relation to the animal has been 

 a subject of discussion. It would appear from the paper of 

 Lacaze Duthiers* that they are anterior and posterior, being 

 articulated across the back, and consequently both dorsal. I 

 have elsewhere called them haemal and neural respectively, as 

 more expressive and correct terms than dorsal and ventral. 

 This opinion had been previously expressed by Huxley, Han- 

 cock^ Jeffreys and others. This relation, however, may not be 

 constant throughout the group, as, for instance, in Lingula. 



The embryos of Thccidium are divided into segments like those 

 of some Pteropods, Dentalium and Chiton. Nothing of the 

 kind, however, is indicated in Morse's notes on the embryonic 

 stages of Terebratulina caput- serpentis.% 



Prof. Morse at one time put forth the theory§ that the pedun- 

 culated or middle dorsal end of the Brachiopods was the ante- 

 rior end, but this being opposed to all known anatomical and 

 embryological facts he has since withdrawn it. 



One character common to all the Brachiopods which I have 

 examined, and perhaps universal, consists in having a projection 

 of the posterior oral labium fitting into a depression in the ante- 

 rior labium, and much resembling the " epistome" in Polyzoa. 

 This is a character of apparently little structural importance, 

 however, and further observations are necessary to establish its 

 constancy throughout the class. 



The relations of this class with the Polyzoa are unquestion- 

 able, and may be compared to the relation existing between the 

 Cephalopods and Gasteropods in the typical Mollusca, while the 

 Tunic ata, on the one hand, and the Lamellibranchiata, on the 

 other, stand somewhat aloof, || and are perhaps closer to each other 

 than any other two classes of the respective subtypes. 



* Annales Sci. Nat. 4th ser. Zool. xv, p. 332, 1861. 



f Trans. Roy. Soc. 1858, p. 851. 



% Am. Naturalist, Sept. 1869, p. 385. 



$ Comni. Essex Inst, iv, p. 173, 1865. 



|| Lacaze Duthiers (Comptes Rendus, Nov. 6, 1865, p. 800) remarks : 

 "Do we not Und an Ascidian (Chevreulius = Rhodosoma or Schizascus) 

 presenting symmetrical muscles analogous to those of the Terebratulce, 

 and this merely because its tunic has become bivalve, and without our 



