574 



BRACHIOPODA. 



and ventral mesenteries, and by the development of the coelom as 

 enteric outgrowths, they recall the Chaetognafha. The possible 

 affinities of the group with Plioronis, in favour of which there is 

 much to be said, are discussed at the end of the chapter. 



The mouth is placed in a buccal groove, one side of which (the 

 dorsal) is provided with a lip, and the other with a row of tentacles. 

 The groove is either continued as a circle on the dorsal lobe of the 

 mantle or is drawn out on each side into the characteristic arms 

 which project into the mantle -cavity. The anus is absent, or 

 present usually on the right side of the front surface of the body, 

 in a position which is described as ventral to the mouth (in Crania 

 it is median, slightly dorsal and posterior). The two valves of the 

 shell are commonly described as being dorsal and ventral, though it 

 must be borne in mind that there are embryological reasons for 

 regarding the surface between the mouth and anus as dorsal, and 

 the whole of the surface on which the shell-valves lie as ventral. 

 There is a circumoesophageal nerve-collar, not separated from the 

 ectoderm and bearing supra-oesophageal and suboesophageal gangli- 

 onic swellings. There are no organs of special sense in the adult, 

 though the larva possesses eye spots. The edges of the mantle- 

 lobes, which are folds of the body-wall, contain setae embedded in 

 ectodermal pits. There is a heart on the so-called dorsal side of 

 the stomach giving off vessels. The perivisceral cavity is coelomic, 

 and extends into the mantle-folds ; it communicates with the exterior 

 by one pair (in Eliynclionella two pairs) of nephridia ; the generative 

 cells are developed on its walls and dehisced into it, and carried 

 outwards by the 



nephridia. Asexual 

 reproduction and par- 

 thenogenesis are un- 

 > known in the group. 

 It is commonly said 

 of the Bracliiopoda 

 that they are pre- 

 eminently an ancient 

 group, and some im- 

 portance is attached 

 to this fact. It is 

 true that they are 



B 



FIG. 457. Rliynchonella psittacea. A, from above ; B from the 

 left side (after Lang). 



an ancient group, representatives of it being known in the Cambrian, 

 and the number of fossil forms is vastly greater than that of the 



