PHYLUM MOLLUSCOIDEA 163 



Much of the space within the shell is occupied by a pair of 

 conspicuous, spirally coiled arms or lophophores. In the order 

 Articulata, the dorsal valve frequently bears a calcareous loop 

 which supports the lophophore. Each arm on its outer margin 

 bears a longitudinal groove bordered by a row of small tentacles. 

 Water currents produced by the ciha on the tentacles and in the 

 groove carry food particles toward the mouth. 



The U-shaped digestive tract consists of a mouth opening in 

 the middle of the lophophore, a dorsally directed gullet which 

 empties into an expanded stomach, and from this a ventrally 

 directed intestine which ends blindly except in members of the 

 order Inarticulata. Cilia line the entire digestive tract. 



Two transverse septa divide the coelom into three somites, but 

 the shortening of the chief axis of the body has been accompanied 

 by a coiling of the digestive tube and consequently the arrange- 

 ment of the septa is somewhat confused and difficult to observe. 

 The coelomic cavities extend into the arms and the mantle 

 lobes. One or two pairs of nephridia communicate with the • 

 coelomic pouches and serve as both excretory and reproductive 

 ducts. The gonads are borne chiefly in the coelomic cavities of 

 the mantle. The sexes are usually separate. 



The brachiopods pass through a trochophore larval stage. 

 During cleavage and embryonic development the eggs are held in 

 brood pouches. At the time of its liberation the larva is divided 

 into three segments. The fully grown larva becomes attached 

 by its posterior end and from this region the peduncle develops. 

 The large midregion or mantle segment of the larva increases in 

 size and secretes the shell characteristic of the adult. 



Brachiopods are exclusively marine. Though represented by 

 relatively few living species, they reached an extreme state of 

 species formation in the Silurian and Devonian periods. Tere- 

 bratulina, Terebratula, and Waldeheimia are characteristic 

 modern genera of the Articulata. 



Class Phoronida 



The relations of members of the single genus Phoronis have 

 been much under discussion among zoologists. Wormlike in 

 form, these marine organisms dwell in membranous or leathery 

 tubes. The body is long, cylindrical, and unsegmented, at one 

 extremity bearing numerous ciliated tentacles arranged in the 

 form of a lophophore as characteristic of members of the phylum 



