246 Introduction to Animal Morphology. 



abdominal aorta (Fig. 31, 2'). A part of the body- 

 cavity surrounding the heart is called the pericardium. 

 Some regions of the integument, either the mantle 

 itself, or a single or double pair of folds within the 

 mantle lobes, or an area of the skin of the back, be- 

 come spongy and dilated as breathing organs, or gills. 

 In land molluscs the cavity within the mantle lobes, 

 or part of it, is set apart as an air-breathing space or 

 pulmonary cavity ; but the breathing organs are never 

 combined with the pharynx, and are always ciliated. 



One or two pair of excretory organs (modified seg- 

 mental organs) commonly exist, opening both on the 

 surface and into the body cavity ; these maybe simple 

 tubes, or may have glandular walls. When largely 

 developed, they receive blood from the viscera, and 

 transmit it directly to the gills, forming a rudimental 

 portal system. The outer openings of these organs 

 may receive the ducts of the sexual organs, or may 

 open in common with them, or may be separate. Other 

 renal organs, special developments of the vascular 

 system, exist in Cephalopoda and Brachiopoda. 



Molluscs are usually hermaphrodite, with a single 

 sex gland (often placed close to the liver), some of 

 whose acini produce ova, others spermatozoa. The 

 male and female products may be emptied by a com- 

 mon duct, or the oviduct and vas deferens may arise 

 in common, and then separate, and finally reunite, or 

 the primary common part may be short or absent.. 

 Various accessory organs are appended in some 

 classes. Development is attended with some form of 

 metamorphosis. The ova are holoblastic, except in 

 Cephalopoda. 



Mollusca are mostly marine ; a few are pseudo- 



