Nervous system and sense-organe
The most primitive nervous system occurs in the monoplacophorans and amphincurans (e.g. chitons, where it consists of a central nerve ring round the anterior oesophagus with pleurovisceral and pedal cords running back, the former encircling the margin of the mantle, the latter running down the foot, with cross connections between the two. Nerve cells lie largely within the cords and are not concentrated into ganglia as they are in the other molluscan classes, which typically possess paired cerebral, pleural, pedal and visceral ganglia, that is ganglia associated with the four regions of the body.
Sense-organs are primitively located in the head where tentacles carry eyes and are also probably concerned with chemoreception. Statocysts (organs of balance) occur in the foot near the pedal ganglia.
Mantle cavity and respiration
Space between the edge of the mantle and the underlying tissues constitutes the mantle cavity. Originally this probably consisted of a groove between the mantle skirt and the foot into which the anus opened posteriorly and the paired kidneys and reproductive system opened laterally; the groove also contained paired series of gills. This is the pattern in the monoplacophoran Neopilina illustrated on page 82. However, it is clear that with further evolution the groove enlarged to form a combined respiratory and cloacal cavity at the posterior end into which the foot could be withdrawn. This is certainly the basic pattern of the respiratory cavity in the majority of molluses. In it the highly characteristic molluscan gills or ctenidia lie protected.
Each ctenidium consists of an axis containing blood vessels, muscle and nerve and bears on each side alternately disposed rows of elongated filaments, Between these, upward-beating rows of cilia create a current of water in the direction opposed to the blood flow within the thin-walled filaments. Efficient respiratory exchange is thus ensured. Associated with the paired ctenidia are osphradia, sense-organs situated near the opening of the cavity and, at least primitively, probably concerned with estimating the amount of sediment carried in with the respiratory current created by the cilia. Hypobranchial glands on the roof of the cavity produce mucus which entangles sediment.
Reproductive system
Reproductive tissue lines forward extensions of the body cavity which are initially in free com munication with the pericardium into which the sexual products are discharged before being conveyed to the exterior through the kidneys. The general effect of evolution within the molluscs is to separate the reproductive from the excretory system with provision of separate ducts. Although most often of separate sexes, there is a strong tendency for hermaphroditism to occur in a variety of forms, especially in the bivalves and the gastro- pods; protandry (a male condition preceding a female one) is particularly common. Most primitively, egg and sperm are liberated for fertilisation to occur in the sea but the higher gastropods and the cephalopods have evolved complicated methods of internal fertilisation. In the former these have made land colonisation possible.
Excretory system
Primarily this consists of kidneys which open internally into the pericardium and externally into the mantle cavity, where the dissolved nitrogenous and other wastes are carried away in the exhalant stream. Most usually a single pair, they may be more numerous when there is more than one pair of auricles, as in the monoplacophorans and in the primitive cephalopod Nautilus. Throughout the gastropods there is only a single functional kidney which, as well as controlling blood concentration, removes waste products of metabolism from circulating blood. Other organs may assist in excretion, for example pericardial glands in bivalves and regions of the digestive diverticula, notably in the cephalopods.