i
C
B
D
FIQ. 69. Respiratory currents in the Cranchiidae. A, Diagram to show the structure of
a cranchiid: inhalent openings (i) at each aide of the head allow water to pase into
paired dorsal mantle cavities (r is the right one) which lie either side of the fluid
filled coelom (c) and are separated from the single ventral mantle cavity (v) by a
thin transverse membrane (t). A spiracle ( 8 ) i n front of the gill connects the
dorsal mantle cavity of each side with the ventral cavity. The funnel (f) opens
from the ventral mantle cavity t o the exterior. B. Water is drawn into the dorsal
mantle cavities by contraction of the anterior end of the coelom. This leads to a
compensctting expansion of the posterior end of the coelom and a pressure on the
water in the ventral mantle cavity resulting in water issuing from the funnel.
C, the coelomic contraction then passes posteriorly, simultaneously pushing water
through the spiracles into the ventral mantle cavity. At the same time the expansion of the anterior end of the coelom pushes thin membranes over the inhalent
openin@ and act aa ValVm to prevent Water from paasing back out of these
openings as water is forced through the spiracles by the expanding anterior coelom.
D, the anterior coelom expands &B the Wave of contrection peaees beyond the end of
the transverse membrane.