312 Mollusca. 



The pleural "■anglion in Lamellibranchs is almost alwavs 

 fused with the cerebral ganglion, and the connective between the 

 pleural and pedal ganglia is fused with that between the cerebral 

 and pedal, so that to all appearance the pleural ganglion and its cord 

 are absent (Fig. 237 D). In some forms, however, the two are 

 independent ; the cord only partially so ; which gives conditions 

 closely approaching to those of the Gastropoda. Byes are only 

 present occasionally, and then are always situated, often in large 

 numbers, along the edge of the mantle ; in the siphonate forms, at 

 the tips of the syphons. For example, in the Scallops [Pecten) 

 there is along the edge of the mantle a series of eyes of somewhat 

 complex structure. An olfactory organ, a specially modified 

 portion of the epidermis supplied with nerves, corresponding with 

 that of the Gastropoda, is present in many, near the anus. 



The mouth is a transverse slit at the anterior end of the body, 

 below the anterior adductor muscle. It is bounded above and below 

 by an upper and lower Up respectively, each of which is drawn out on 

 both sides into a usually well-developed labial palp. The labial 

 palps are covered with numerous cilia ; these serve to drive into the 

 mouth the small particles, microscopic plants and animals, which are 

 present in the water taken into the mantle-cavity. R a d u 1 a and 

 jaws are absent. A short oesophagus leads from the mouth into a 

 stomach, which is often provided with a c se c u m ; in the latter 

 is contained the so-called crystalline stylet, a gelatinous, 

 transparent body which is secreted by the epithelium of the ceecurn, 

 developing when food is plentiful (in the summer), and redissolving 

 during scarcity (in winter) ; it probably represents a reserve of 

 nourishment, and is present in almost all Lamellibranchs.* A 

 well-developed liver surrounds the stomach, and opens into it by 

 several apertures. The intestine is coiled several times ; its posterior 

 portion runs along the dorsal side of the animal, and finally passes 

 dorsal to the posterior adductor muscle to open at the hind end of 

 the body. The ventricle is situated on the dorsal side of the 

 animal above the rectum ; it generally divides into two branches which 

 surround the gut and unite below it, so that it is ring-shaped, 

 "perforated by the rectum." There are two auricles, one on each 

 aide, which receive the blood from the gills and carry it to the 

 ventricle. The vascular system is imperfect ; in the gills there is, 

 however, a rich network of capillaries. The kidneys are a pair of 

 sac-like organs which are often more or less intimately connected 

 with each other (the organs of Bojanus) ; each opens by an aperture 

 situated laterally below the origin of the inner gills ; and also into 

 the pericardium. Most of the Lamelhbranchs are of separate 

 sexes, a few {e.g., the Oyster) are hermaphrodite. Ovary and 



* A crystalline style is also present in some Gastropoda. 



