228 MANUAL OF ZOOLOGY. 



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 some cases, intervenes between the arteries 

 and veins, appears from recent researches to be probable. 



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. 



The nervous system varies considerably in its development. 

 In the Polyzoa, Tunicata, and Brachiopoda which collectively 

 constitute the Molluscoida the nervous system consists of a 

 single ganglion, or of a principal pair with accessory ganglia, 

 placed between the oral and anal apertures, or on the ventral 

 surface of the body. The true Molluscan type, however, of 

 nervous system is constituted by the presence of three pairs of 

 ganglia, connected with one another by commissures, but 

 distributed in a characteristically scattered manner (hetero- 

 gangliate type). One of these ganglia is situated above the 

 oesophagus, and is called the ' supra-oasophageal,' or ' cerebral' 

 ganglion. A second is placed below the oesophagus, and is 

 termed the ' infra-oesophageal,' or 'pedal' ganglion (from its 

 supplying the nerves to the 'foot'). The third pair is the 

 most persistent, and is termed the ' branchial,' or ' parie to- 

 splanchnic ' ganglion. 



