366 ZOOLOGY SECT. 



to the mantle, lophophore, &c. No special sense-organs are 

 known. 



Reproductive Organs. The sexes are separate. There are 

 two pairs of gonads (Fig. 290, gon), one dorsal and one ventral, in 

 the form of irregular organs sending off branches into the pallial 

 sinuses. 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Brachiopoda are Molluscoida in which the body is enclosed 

 in a shell formed of two parts or valves which are respectively 

 dorsal and ventral in position. The body occupies only a small 

 portion of the space enclosed by the shell, and is usually attached 

 to foreign objects by a posteriorly placed stalk or peduncle : it 

 gives off dorsal and ventral reduplications, the mantle-lobes, which 

 line the valves of the shell and enclose a large mantle-cavity. 

 From the anterior surface of the body is given off a lophophore 

 which surrounds the mouth, and is beset with ciliated tentacles. 

 There is a ridge-like pre-oral lip which is continued on to the 

 lophophore. The enteric canal is usually V-shaped, and is 

 divisible into gullet, stomach, and intestine : there is a pair 

 of digestive glands. The ccelome is spacious, and is continued 

 into the mantle-lobes. A heart is usually present, attached to 

 the stomach. The excretory organs are one or two pairs of 

 nephridia which act also as gonoducts. The nervous system is a 

 ganglionated circum-cesophageal ring : sense-organs are usually 

 absent in the adult. The sexes are separate or united. Develop- 

 ment is accompanied by a metamorphosis. 



The class is divided into two orders : 



ORDER 1. INARTICULATA. 



Brachiopoda in which the shell is not composed of oblique 

 prisms : the valves are not united by a hinge, and there is no 

 shelly loop for the support of the lophophore. An anus is 

 present. 



Including Lingnla, Crania, Discina, &c. 



ORDER 2. ARTICULATA. 



Brachiopoda in which the shell is formed of oblique prisms or 

 spicules of calcium carbonate : the two valves unite by a definite 

 hinge, and there is usually a shelly loop, for the support of the 

 lophophore, developed in connection with the dorsal valve. The 

 intestine ends blindly. 



Including Magellania, Terebratula, Rhynchonella, Cistella 

 (Argiope), &c. 



