182 INVERTEBRATE ANIMALS. 



A distinct heart is almost always present, and consists of 

 two chambers, an auricle and a ventricle. Respiration is very 

 variously effected one great division being constructed to 

 breathe air by means of water, while in another section the 

 respiration is aerial. In the former of these often spoken of 

 as the " branchiate " Gasteropods respiration may be carried 

 on in three ways : Firstly, there may be no special breathing- 

 organ, the blood being simply exposed to 

 the astion of the water, as it circulates 

 through the thin walls of the mantle-cavity. 

 Secondly, the breathing-organs may be in 

 the form of outward processes of the skin, 

 exposed to view on the back or sides of the 

 animal (Fig. 85). Thirdly, the breathing- 

 organs are in the form of plume-like gills, 

 contained in a more or less complete cham- 

 ber, formed by a folding of the mantle. In 

 many members of this group the water ob- 

 tains access to the gill-chamber by means of 

 FIG. 83. Portion of the a tubular prolongation or folding of the 



mantle formin s a ^p 1 ? 011 ( Fi s- 82 ' *) and 



Wood " ^ en ^e e ff ete water is expelled by another 

 tube which is similarly constructed. In the 

 second great section often called the "pulmonate" Gaste- 

 ropods respiration is effected by a pulmonary chamber or lung, 

 formed by a folding of a mantle, and having air admitted to 

 it by a distinct aperture. 



The sexes in the Gasteropoda are mostly distinct, but they 

 are sometimes united in the same individual. The young, 

 when first hatched, are always provided with an embryonic 

 shell, which may be entirely lost in the adult, or may simply 

 become concealed by a fold of the mantle. In the water- 

 breathing forms the young is protected by a small nautilus- 

 shaped shell, within which it can entirely withdraw. It is en- 

 abled to swim about freely by means of two ciliated lobes spring- 

 ing from the sides of the head, and in this stage it is very 

 like the permanent adult condition of the Pteropoda (Fig. 88). 



As regards the shell of the Gasteropoda, the following 

 points may be noticed: The shell is composed either of a 

 single piece (univalve), or of a number of plates placed one 

 behind the other (multivalve). 



The univalve shell is to be looked upon as essentially a 

 hollow cone, the apex of which is placed a little on one side. 

 In the simplest forms, as in the Limpets, the conical shell is 



