162 



INVERTEBRATE ZOOLOGY 



and from the posterior end of the animal a stalk or peduncle for 

 attachment is frequently developed. The valves are lined by a 

 mantle by whose action the shell is secreted. The bristles borne 

 in the edge of the mantle are of the same type as those found in 

 annelids and seem to point to a relationship between brachiopods 

 and annelids. The ventral shell in many cases bears a short 

 beaklike projection posterior to the hinge and it is through this 

 that the peduncle passes. The shells so closely resemble the 

 most primitive type of oil lamp that the common name "lamp 

 shell" is very generally applied to shells of this group. 



Manfle lobe 



Lophophore 

 Doiiol valve 



Gonad 



Di'gesh've gland 



Shmach 

 Heart 



Ventral 



Mouth 



Mantle 



Fig. 81. — Semidiagrammatic sagittal section of a brachiopod {Magellania 

 lenticularis) . {Redrawn from Parker and Haswell, with the permission of Mac- 

 m.illan Co.). 



In members of the order Inarticulata (Lingula, Crania, and 

 Discina), the foregoing description does not apply, for the two 

 valves are similar and the hinge is wanting. In the Articulata, 

 where a hinge is present, the valves are not held open by an 

 elastic hinge ligament as in the Acephala of the molluscs, but 

 both opening and closing of the valves are accomplished by 

 muscular action. Closure of the shell is by means of a pair of 

 adductor muscles which are attached to the dorsal shell but unite 

 to form a single muscle before reaching their insertion on the 

 ventral valve. Two pairs of divaricator muscles pass between 

 the ventral valve and that part of the dorsal valve posterior to the 

 hinge. By their contraction the valves are opened. From each 

 valve a pair of muscles known as the adjustors pass to an insertion 

 on the peduncle. It is through contraction of these last muscles 

 that the animal is able to shift the position of the entire body. 



