202 



INVERTEBRATE ANIMALS. 



stomach, which sometimes is provided with calcareous plates for 

 grinding down the food. The intestine is long, and always ter- 

 minates in a distinct anal aperture. Distinct salivary glands are 

 usually present, and the liver is well developed. 



A distinct heart (fig. 1 29) is almost always present, and consists 

 (if two chamliers, an auricle and a ventricle. Respiration is very 

 variously effected — one great division being constructed to breathe 



Fig. 140. — A, Sketch of a Whelk (BvcHnum undatum) in motion ; / Foot ; h Head, 

 carrying the feeler.s (t) with the eyes (e) .it their bases ; p Proboscis : s Respiratory 

 siphon or tube by which water is admitted to the frills ; o Operculum. B. Shell of 

 the Whelk : a Spire ; h Body-whorl n Notch in the front margin of the mouth of 

 the shell ; m Outer lip of the mouth of the shell. This figure is half the natural 

 size. C, A small cluster of the egg-capsules of the whelk. (B and C are after 

 Woodward.) 



air by means of water, whilst 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 

 ex])osed to the action 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. 144). Thirdly, the breathing- 



