An Introduction to the Invertebrates, Second Edition - tiera.ru

140 MOLLUSCA: BIVALVIA AND CEPHALOPODAFig.11.3 The structure ofscaphopods: (a) external view ofDentalium; (b) diagrammaticlongitudinal section of Dentalium.shoals. Cephalopods also include the largest invertebrates: the bodyof a giant squid may be 15 metres long.11.6 How is the molluscan body plan modifiedin Cephalopoda?Whether the shell is retained, as in Nautilus, or internal and reducedor lost, as in coleoids, the animals are expanded along thedorsoventral axis and at the same time the ventral side becomesanterior (Figure 11.4a). The mantle cavity is ventral and opensanteriorly (behind the head). Part of this opening is filled by thefunnel, which represents part of the foot, through which water isexpelled in jet propulsion (see below). The ctenidia lack cilia, beingventilated by the stream of water drawn through the mantle cavityby contractions of the mantle wall. There is a closed blood circulationin place of an open haemocoel. The foot is represented by thefunnel and by the muscular tentacles (arms) round the mouth. Preyis seized by the arms, masticated by beak-like jaws and transportedfurther by the radula. Eyes are large and prominent.11.7 What cephalopods are known?Ammonites. These well-known and commonly found fossilsare among a variety of extinct groups with external shells