MOLLUSCOIDA : BRACHIOPODA. 247 



and also to convey away the reproductive elements, the organs 

 for which are developed in various parts of its walls. By 

 Woodward, the pseudo-hearts are regarded as oviducts, and it 

 is stated that they have been found to contain mature ova, so 

 that there can be little doubt but that this view of their nature 

 is the correct one. 



The function of respiration is probably performed, mainly, 

 if not entirely, by the cirriferous oral arms, as it appears 

 chiefly to be by the homologous tentacular crown of the 

 Polyzoa. A true vascular system and a distinct heart are 

 present in some, at any rate, of the Brachiopoda, but this 

 subject is still involved in considerable obscurity. In Tere- 

 Iratula the heart is in the form of a unilocular, pyriform 

 vesicle, placed on the dorsal surface of the stomach. 



The nervous system consists of a principal ganglion, of no 

 great size, placed in the re-entering angle between the gullet 

 and the rectum. 



The sexes are most probably united in the same individual. 

 Little is known of the development of any of the Brachiopoda, 

 but the embryo is doubtless free-swimming and locomotive. 



CLASSIFICATION OF THE BBACHIOPODA. (AFTER DAVIDSON.) 



CLASS BBACHIOPODA. 



Fam. I. Terebratulid<e. 



Shell minutely punctate ; ventral valve with a prominent beak 

 perforated by a foramen for the emission of a muscular peduncle, 

 whereby the animal is fixed to some solid object. Foramen partially 

 surrounded by a deltidium of one or two pieces. Oral appendages 

 entirely or partially supported by calcified processes, usually in the 

 form of a loop, and always fixed to the dorsal valve. 



Genera. Terebratula (with Terebratulina, and WaldJieimia), Tere- 

 bratella, Stringocephahis, &c. 

 Fain. II. Thecididce, 



Shell fixed to the sea- bottom by the beak of the larger or ventral 

 valve ; structure punctated. Oral processes united in the form of a 

 bridge over the visceral cavity ; cirrated arms folded upon themselves, 

 and supported by a calcareous loop. 



Genus. Thccidium, 

 Fam. III. S^iriferidts. 



Animal free, or rarely attached by a muscular peduncle. Shell 

 punctated or unpunctated. Arms largely developed, and entirely sup- 

 ported by a thin, shelly, spirally-rolled lamella. 



Genera. Spirifer, Spiriferina, Cyrtia, Athyris, $c. 

 Fam. IV. Koninckida. 



Animal unknown. Shell free; valves unarticulated (?). Oral arms 

 supported by two lamellae, spirally-coiled. 



Genus. Koninckia. 

 Fain. V. RkynchonelUdtB. 



Animal free, or attached by a muscular peduncle issuing from an 

 aperture situated under the extremity of the beak of the ventral valve. 



