xii PHYLUM MOLLUSCA 695 



serve to carry food-particles to the mouth : the enteric canal is 

 coiled, and is formed mainly from the mesenteron : there are large 

 paired digestive glands : the rectum passes through the pericardium, 

 usually perforates the ventricle, and ends above the posterior 

 adductor. The heart is contained within the pericardium, and 

 consists of a median ventricle and of right and left auricles : the 

 blood, which is usually colourless, is taken from the ventricle to 

 the body by one or two aortse, and is returned partly directly, 

 partly by way of the renal organs and gills, to the auricles. The 

 renal organs are a single pair of ccelomic nephridia, which 

 usually open at one end into the pericardium, at the other on the 

 exterior. The nervous system consists typically of four pairs of 

 ganglia called respectively cerebral, pleural, pedal, and visceral : 

 the cerebral and pleural of each side are usually fused into a single 

 cerebro-pleural ganglion. The chief sense-organs are statocysts 

 and osphradia or water-testing organs. The sexes are separate or 

 united : there are no accessory organs of reproduction. Develop- 

 ment is accompanied by a metamorphosis, which usually includes 

 a trochophore stage. 



The classification of the Pelecypoda is as follows : 



ORDER 1. PROTOBRANCHIA. 



Pelecypoda in which the gills take the form of a single pair of 

 plume-like organs or ctenidia, each with two rows of flattened 

 gill-filaments. The foot is not compressed, but has a flattened 

 ventral surface or sole upon which the animal creeps. There are 

 two adductor muscles. 



This group includes only four genera Nucula (Fig. 586), 

 Yoldia, Leda, and Sohnomya. 



ORDER 2. FILIBRANCHIA. 



Pelecypoda in which there is a pair of plate-like gills formed 

 of distinct V-shaped filaments: interfilamentar junctions are either 

 absent or formed by groups of interlocking cilia : interlamellar 

 junctions are either absent or non- vascular. As a rule there are 

 two adductor muscles, but the anterior may be greatly reduced or 

 absent. 



Including the Noah's ark shell (Area), Sea-mussel (Mytilus, 

 Fig. 585), Anomia, Trigonia, &c. 



ORDER 3. PSEUDO-LAMELLIBRANCHIA. 



Pelecypoda in which the gills are plaited so as to present 

 vertical folds: the interfilamentar junctions may be ciliary or 

 vascular : the interlamellar junctions vascular or non-vascular. 



