154 MOLLUSCA. 



Class I. BRACHIOPODA. 



SPIROBRANCHIATA. PALLIOBRANCIIIATA. HAPLOCARDIA. 



Headless, symmetrical mollusks, enclosed in a bivalve shell. 

 Mouth with two long, cirriferous arms. No true branchias. 

 Mostly hermaphrodite. 



The valves of the shell are above and below the animal, not 

 right and left, as in ordinary bivalves, and they have no elastic 

 ligament ; the lower, posterior, or ventral valve is frequently pro- 

 longed into a beak, and perforated, to allow the passage of a pe- 

 duncle by which the animal attaches itself to some foreign body. 

 The dorsal integument lines the interior of the valves, and " forms 

 by its expansions the lobes of the mantle," here subservient to 

 respiration. The heart is a simple ventricle ; the cavity formed 

 by the lobes is partly occupied by " two long fringed arms, con- 

 tinued from the sides of the mouth, and disposed in folds and 

 spiral curves." 



Most of the species (upwards of 1800) are extinct ; they appear 

 to have been most abundant in the Silurian epoch ; about 80 

 still exist, some living at a depth of 2500 fathoms. 



Brachiopoda are embryologically allied to the Vermes, and by 

 some writers they are cousidered to be very similar in structure 

 to the Polyzoa. Ray Lankester unites them with the Lamelli- 

 branchiata, under the name of Lipocephala. Glaus places them 

 as a pendant to the Mollusca. They are the lowest " stage of 

 genuine mollusks," according to Hackel ; but others consider that 

 they are most allied to Annelida. Huxley holds their affinities 

 to be between the Polyzoa and the higher Mollusca. 



There are two orders : 



Shell hingeless LYOPOMATA. 



Shell with a hinge-line AKTHROPOMAT A. 



Order I. LYOPOMATA. 



ECARDINES. PLEUROPYGIA. INARTICULATA. SARCOBRANCHIATA. 



Valves not toothed, held together by muscles ; shell corneous. 

 Intestine terminating in a lateral anus. 



The oral arms are mostly fleshy, and without the bony support 

 of the next order. In the Craniidse the shell is calcareous ; in 

 the Lingulidge it is covered with a horny epidermis ; in the latter 

 the peduncle is emitted from between the valves. 



