Xtl PHYLUM MOLLUSCA 6?7 



which is usually ploughshare- or tongue-shaped : in some cases 

 there is a byssus-gland posterior to the foot, which secretes a mass of 

 horny fibres, the byssus, by which the animal may be permanently 

 attached. There are two gills or ctenidia, one on each side : the 

 chief function of the gills is the production of a respiratory and food- 

 carrying current of water. The body is covered by a one-layered 

 epidermis, which is ciliated on the gills and on the inner surface of 

 the mantle. The muscular system is well-developed, the largest 

 muscles being either one or 'two adductors, which close the shell, 

 and several bands connected with the foot and byssus ; the 

 muscles are usually unstriped. The ccelome is reduced to a 

 dorsally-placed pericardium. The mouth 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 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 coelomic kidneys, 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 : 



OEDEB 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. 596), Yoldia, 

 Leda, and Solenomya. 



ORDER 2. FILIBRANCHIA. 



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

 of distinct V-shaped filaments : interfilamentar junctions are either 



