14 The Molluscan Family Plauorbidae 



supplied with oxygen, in company with Lymnaeas and other fresh water 

 piihnonates. The pseudobranch is probably helpful in supplying oxygen 

 to the blood, as it is highly vascular (plate 14, fig. 4, P). 



[e] The Respiratory System 



Respiration in the Planorbidae is effected as in the Lymnaeidae and other 

 aquatic pulmonale mollusks. Free air is admitted into a large mantle 

 cavity abundantly lined with blood vessels forming a plexus on the dorsal 

 wall of the mantle cavity. This lung cavity lies on the right side of the 

 mollusk, next to the kidney, and is bordered on the left by the pulmonary 

 vein (plate 44, fig. 2, L). The lung cavity is placed on the right side of the 

 body in sinistral species iPlanorbis, Physa) but on the left side in dextral 

 species (Lymnaea). 



In the Planorbidae the presence of the secondary brancliium, the 

 pseudobranch, enables the animal to respire without coming to the surface. 

 This organ is of various shapes, but is always an outgrowth of the 

 branchial cavity walls and lies to the left of the pulmonary siphon to 

 which it is attached (plate 14, figs. 3, 4, P). It may be a simple rounded 

 fold or it may be flat with a distinct ridge down its center. It is abundantly 

 supplied with a plexus of blood vessels which absorb oxygen from the 

 water. 



The rectum passes through the pseudobranch and its outlet (anus, A) 

 is usually above and about at the middle of the pseudobranch. The rectum 

 usually has a large crest or ridge on its upper surface which diminishes 

 in size at the lower part where it extends over and to the edge of the 

 pseudobranch (see plate 25, fig. 3, Helisoma pilsbryi; plate 28, fig. 8, 

 Hclisoma scalare; plate 35, fig. 4, Carinifex jacksonensis) . 



The breathing orifice or pulmonary siphon (pneumostome) is an out- 

 growth of the mantle which is capable of forming a tubular conduit or 

 siphon through which free air may be admitted to the lung (see the 

 plates mentioned above). The respiratory, renal, and circulatory systems 

 are intimately connected and should be considered together to understand 

 the conditions governing the respiration and blood circulation of the 

 animal. 



[f] The Nervous System 



The nervous system of the family Planorbidae differs little from that of 

 the family Lymnaeidae (see Baker, 1911, plates 1 and 5). A nerve ring 

 surrounds the oesophagus posterior to the buccal sac (plate 48, fig. 18). 

 There are two cerebral ganglia above the oesophagus (CG). Below the 

 oesophagus there are two pedal ganglia (PG), two buccal ganglia (BG), 

 two visceral ganglia, and a single abdominal ganglion. Two pleural ganglia 

 are connected with the visceral, pedal, and abdominal ganglia. The left 

 cerebral ganglion sends nerves to the penial complex (plate 48, fig. 17, 

 Helisoma anceps) , which divide to send branches to the vergic sac, penial 

 gland duct region, and retractor muscle, as well as to other parts of this 

 comj^lex. 



In the Planorbidae these nerves are sent off from the left instead of the 

 right ganglia, because the group is sinistral. In the Lymnaeidae, a dextral 

 group, the nerves arise from the right ganglia (see Baker, 1911, plate 5). 



