vin PHYLUM MOLLUSCOIDA 337 



The excretory organs consist of a pair of very large nephridia 

 (nph) lying one on each side of the intestine. Each is funnel- 

 shaped, having a wide inner opening or nephrostome, with plaited 

 walls, opening into the coelome, and a narrow curved outer portion 

 which opens into the mantle-cavity not far from the mouth. As 

 in many cases which have already come under our notice the 

 nephridia act also as gonoducts. 



The nervous system is a ring (Fig. 269) round the gullet pre- 

 senting supra- (g) and infra- (usg) oesophageal swellings or ganglia 

 of which the infra-oesophageal is the larger. Nerves are given off 

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

 known. 



Reproductive Organs. The sexes are separate. There are 

 two pairs of gonads (Fig. 266, gori), one dorsal, 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 

 il 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 prse-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 coelome 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-oesophageal 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 Lingula, Crania, Discina, etc. 



VOL. I Z 



