646 ZOOLOGY SECT. 



is bounded by two pairs of flat, triangular tentacles or labial palps 

 the cilia of which 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 peri- 

 cardium, 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 aortas, 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 meso-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 otocysts and osphradia or 

 water-testing organs. The sexes are separate or united : there 

 are no accessory organs of reproduction. Development is accom- 

 panied by a metamorphosis, which usually includes a trochosphere 

 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 N-uculfi, Yoldia, Lcda, 

 and Solenomya. 



ORDER 2. FILIBRANCHIA. 



Pelecypoda, in which there are two pairs 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 11011- 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), 

 &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. 



