614 MOLLUSCA GENERAL DESCRIPTION. 



or x l^'l'i'l* 5c, is the formula for certain kinds. Further allusion 

 to these will be made when treating of the different groups. 



All the Mollusca have a circulatory system, with a heart as the central 

 organ, which propels the blood through the body. The heart lies in a cavity, 

 known as the pericardium, and receives from auricles, usually two in number 

 and lying one on each side of it, the blood that has been aerated in the gills. 

 This blood is driven by the heart through a series of arteries, whence it 

 passes in most Mollusca through a series of cavities amongst and between 

 the various organs and muscles of the body ; hence it finds its way back, for 

 the most part through veins, to the gills, but some passes direct to the heart 

 without aeration and thence circulates again. The blood is often colourless, 

 or is slightly bluish, from the presence of an albuminoid (hwmocyanin) con- 

 taining copper ; sometimes it is red, and contains the same colouring matter 

 (hcemoylobin) as the blood of vertebrates. 



The gills have been so largely employed in classifying Mollusca 

 that it is impossible to omit a brief description of their main 

 features. The simple or primitive form of gill has been aptly 

 compared to a plume, in that, like an ostrich feather, it con- 

 sists of a main axis or shaft, from which, at short intervals, filaments are 

 given off in pairs. The axis of the gill contains, on 

 opposite sides, the vessels that convey the blood 

 to the smaller passages in the filaments and carry 

 it off again to the heart after it has been aerated 

 in them. Increase in the surface of aeration is 

 obtained either by the lengthening of the fila- 

 ments, or by the union and flattening out of each 

 pair into a small leaf-like expansion. The former 

 is the Filibranchiate,orPectintinibranchiate (comb- 

 Fig. 2 DIAGRAM ILLUSTRATING gilled), the latter the Scutibranchiate, or Aspido- 

 THB Two TYPES OF GILL branchiate (both meaning shield - gilled), or 

 S Tp T e U ctinibra D chiate. Foliobranchiate (leaf-gilled) condition (Fig 2). 



B, Foliobranchiate. Further modifications take place in the gills of 



bivalves, but these will be treated of later on. 



In the Foliobranchiate, and in the more complicated forms of gill, the 

 vessels conveying the blood to and from the gill have their positions with 

 relation to the axis considerably modified. 



A pair of organs called nephridia, which play the part of kidneys and free 

 the blood from impurities on its way to the gills, lie in the region of the 

 heart ; they open into the pericardium on the one hand, and communicate on 

 the other with the exterior. 



In the majority of Mollusca the sexes are distinct, but in the land snails, 

 the sea slugs, and their allies, the sexes are united in each individual. Most 

 Mollusca are oviparous ; that is to say, lay eggs. In a few instances the young 

 are brought forth alive, the eggs being hatched within the body of the 

 parent ; whilst in numerous other cases, notably the bivalves, the eggs are 

 retained within the parent shell till hatched. The land snails lay beparate 

 eggs, which have each a tough outer layer or case, or even a regular shell 

 like a bird's egg. There is one West African snail whose egg is as large as 

 that of a pigeon, and has nearly as thick a shell. The fresh water and 

 marine snails deposit their eggs generally in clusters, adhering together 

 either in a mass, or spread out in a ribbon. Sometimes each egg is enclosed 

 in a tough albuminous capsule, as in the case of the whelk and the cuttle-fish. 



