XVII THE STALK 



475 



ends of the anterior pair are supported by the gastro-parietal 

 mesenteries, and those of the posterior pair by the ileo-parietal 

 mesenteries. In all other Brachiopods the posterior pair alone 

 exists. Tlie external opening of these nephridia is near the 

 base of the anus ; in Cistella it is at the bottom of a brood-pouch 

 formed by the tucking in of the body wall in this neighbour- 

 hood, and in this brood-pouch the eggs develop until the larval 

 stage is reached. 



The walls of these nephridia are lined by ciliated cells, 

 amongst which are some excretory cells, in which numerous 

 brown and yellow concretions are to be seen ; these are probably 

 the nitrogenous excreta of the animal, and pass out of the body, 

 being washed away by the stream of water which is constantly 

 passing between the shells. 



As in so many other animals, the nephridia act as genital 

 ducts, and through them the ova and spermatozoa, which break 

 off from the genital glands and fall into the body cavity, find 

 their way to the outer world. 



The Stalk and Muscles 



The body cavity of a Brachiopod is traversed by several pairs 

 of muscles, which are very constant in position, and whose con- 

 traction serves to open and close the shell, to move the animal 

 upon its stalk, and to govern the movements of the arms. 



The stalk is absent in Crania^ and the members of this 

 genus are attached to the rocks on which they are found by the 

 whole surface of their ventral valve. In Lingida (Fig. 315) 

 the stalk is long and hollow, containing what is probably a 

 portion of the body cavity, surrounded by muscular walls. 

 Lingida is not a fixed form, but lives half-buried in the sand 

 of the sea-shore (Fig. 321). Discina^ the other member of 

 the Ecardines, has a peduncle which pierces the ventral valve 

 and fixes the animal to its support. Amongst the Testicardines, 

 Thecidium is also fixed to its supports by the surface of its 

 ventral valve ; the other genera, however, are provided with 

 stalks, which, being the means of the fixation of the animals,, 

 become at the same time the fixed points upon which their 

 very limited movements can be effected. The stalk protrudes 

 through the notch or aperture at the posterior end of the ventral 



