57 



short time, when there is no apparent cause for 

 disf urbance. 



(II) Respiratory Mechanism. 



1. Gills. — Eledone, in common with all the living 

 Cephalopods except Nautilus, is Dibranchiate, i.e., it has 

 a single pair of gills. Morphologically these gills 

 represent part of the inner surface of the mantle, which 

 has been specialised for respiratory purposes. They are 

 situated in the mantle cavity, laterally to the visceral 

 mass. When the mantle is cut open, and turned back (as 

 in tig. 11), they maybe seen slanting obliquely outwards 

 and upwards from the posterior end of the visceral mass. 

 In fig. N, which shows the left division of the mantle 

 cavity, only one may be seen. They are attached both to 

 the mantle and to the visceral mass. The gill may be 

 considered as a slender hollow cone, with the apex 

 pointing upwards and outwards. The hollow which forms 

 the base leads into the cavity of the cone, wdiose walls are 

 formed by the branchial leaflets. The cone is, however, 

 laterally compressed, and is attached to the mantle 

 dorsally, but free ventrally. Along this dorsal attached 

 axis of the gill runs the afferent vessel, while along the 

 free one runs the efferent vessel, so that the plane joining 

 these two axes bisects the gill, and is at right angles to 

 the plane of the mantle. The tip of the cone is situated 

 just posterior to the level of the anus, while the base is 

 slightly behind the urinary papilla. Below this may be 

 seen the branchial heart, which, receiving blood from the 

 body, pumps it on through the afferent vessel to the gill 

 tor aeration. Posteriorly the gill is bound to the visceral 

 mass by (1) The two muscular bands, mentioned in the 

 description of the mantle cavity. The first runs down the 

 ventral side of the efferent vessel, from its tip, and then, 



