254 MANUAL OF ZOOLOGY. 



poda)\ and, lastly, the Cephalopoda have, in addition, horny or 

 calcareous mandibles, forming a kind of beak. Well-developed 

 salivary glands are usually present ; the liver in the higher 

 forms is of large size, and pours its secretion either into the 

 stomach or into the commencement of the intestine; and a 

 renal organ has been detected in most of the Mollusca proper. 

 There is no distinct absorbent system, but the products of 

 digestion pass by exosmose into the general abdominal cavity, 

 and thence into the larger veins, which are sometimes pierced 

 by numerous round holes for this purpose. 



The blood is colourless, or nearly so. In the Polyzoa the 

 circulation is carried on by ciliary action, and there is no dis- 

 tinct propulsive organ, or definite course of the circulating 

 fluid. In the Tunicata the heart is a simple tube, open at both 

 ends, and the course of the circulation is periodically reversed. 

 In the Brachiopoda the course of the circulation is not defi- 

 nitely ascertained, and it is doubtful if a true heart is present 

 in all. In the higher Mollusca a distinct heart is always 

 present, and consists of an auricle which receives the aerated 

 blood from the breathing -organ, and a muscular ventricle 

 which propels it through the systemic vessels. That a system 

 of capillaries in many cases intervenes between the arteries 

 and veins, appears from recent researches to be probable. 

 In all cases the heart of the Mollusca is systemic, distributing 

 the aerated blood to the body, and in no case is it respiratory, 

 propelling the non-aerated blood to the breathing-organ. 



In the Polyzoa there is no differentiated respiratory organ, 

 and the function of respiration is discharged mainly by the 

 oral crown of ciliated tentacles. In the Tunicata respiration 

 is effected by means of the pharyngeal or branchial sac ; and 

 in the Brachiopoda by the oral arms, and possibly, to some 

 extent, by an " atrial " or " water- vascular " system, furnished 

 with contractile dilatations. In the higher Mollusca a distinct 

 breathing-organ is always present, a portion of the mantle being 

 specialised for this purpose. In the Lamellibranchiata, and 

 the branchiate Gasteropoda, the breathing-organs are in the 

 form of lamellar and pectinate gills, and the same is the case 

 with the Cephalopoda. In the pulmonate Gasteropoda, in 

 which respiration is aerial, a pulmonary sac or air-chamber is 

 produced by the folding of a portion of the mantle, over the 

 interior of which the pulmonary vessels are distributed. The 

 chamber thus formed communicates with the exterior by a 

 round aperture which can be opened or closed at will ; and the 

 renovation of the effete air within the sac appears to be effected 

 mainly or entirely by simple diffusion. 



