PHYLUM MOLLUSCOIDA 



3(i0 



In the Articulata the enteric canal is V-shaped, as in Magel- 

 lania, the intestine being straight or nearly so, and ending blindlj'. 

 In the Inarticulata, on the other hand, the intestine is usually 

 coiled, and always ends in an anus (Fig. 295, C, a), which generally 

 opens into the mantle-cavity, but in one genus (Crania) into a 

 pouch or sinus at the posterior end of the body between the 

 valves. 



A heart is usually present, but the function of blood is per- 

 formed mainly by the coelomic fluid, which is propelled by the 

 cilia lining that cavity, and circulate both in the ccelome itself and 



Pig. 2!i5.— Dissections of A, Cistella ; B, Rhynctaonella ; and C, Iiingula. a, anus ; Ipli. 

 lophophore ; mth. moutli. (After Schulg^n and HancockJ 



in the pallial sinuses, each sinus presenting — in Lingula at least 

 — both an outgoing and an ingoing current. 



A single pair of nephridia, resembling those of Magellania, 

 occurs in all known genera except Rhynchonella, in which there are 

 two pairs, one dorsal and one ventral. Besides discharging an 

 excretory function they act as gonoducts. 



The nervous system always takes the form of a circum-oeso- 

 phageal ring with ganglionic enlargements, the largest of which 

 is ventral or sub- oesophageal in position. Otocysts have been 

 described in Lingula, rudimentary eyes in Megerlia, and patches 

 of sensory epithelium in Cistella : with these exceptions sensory 

 organs are unknown. 



There are usually four gonads, two dorsal and two ventral, 



VOL. I B B 



