604 MOLLUSCA 



open into the pericardial cavity. The kidneys bear a close relation to 

 the large veins brmging blood to the branchial hearts. 



The main ganglia are massed together around the forward end of 

 the oesophagus and are suiTounded by a protective capsule of cartilage. 

 On the dorsal side of the oesophagus are the cerebral, on the ventral side, 

 lying close together, the visceral (viscero-pleuro-parietal) and the pedal 

 ganglia, which are joined with the cerebral by broad connectives. In 

 front of the pedal are the brachial ganglia, which are genetically a part 

 of them and send off large nerves to the arms. Two pairs of large 

 pharyngeal ganglia, an upper and a lower pair, lie at the base of the 

 pharynx. A pair of large optic ganglia spring from the brain, and a pair 

 of large stellate ganglia, which are joined to the visceral by connectives, 

 lie on the inner surface of the mantle. 



The special sense organs are well developed. The eye is a camera 

 obseura consisting of a deep pit with a retina on its inner surface and 

 a small opening to the outside for the admission of light. In Nautilus 

 the eye is of this form. In other cephalopods, a large leng is present at 

 the opening, and in most forms a transparent cornea is also present in 

 front of the lens. The superficial similarity of this eye to the vertebrate 

 eye is very strikmg. A pair of lithoeysts, which are organs of equilib- 

 rium, lie on the under side of the cerebral cartilage. A pair of pits just 

 behind the eyes are probably olfactoiy organs. Osphradia are present in 

 Nautilus, at the base of the gills. 



The sexes are separate. The gonad is single and occupies most of 

 the upper half of the visceral mass. The ducts leading from it are either 

 single or paired; they do not join the gonad directly, but come from a 

 capsule surrounding it, which is a portion of the ccelom, and extend to 

 the mantle cavity. In the female of most decapods large glandular 

 structures called the nidamental glands open into the mantle cavity near 

 the opening of the oviduct, which secrete the egg capsules. The vas 

 deferens is single (except in Nautilus) and secretes the spermatophores. 

 These capsules, filled with sperm, are introduced into the mantle cavity of 

 the female by one of the arms of the male, which is often specially modi- 

 fied for this purpose, and is called the hectocotylus arm. In Argonauta 

 and one or two other octopod genera, the hectocotylus arm becomes sep- 

 arated from the body and enters the mantle cavity of the female with its 

 load of spermatophores, where it may remain for days, and as it was 

 supposed at one time to be a parasitic wonn, it was given a name by 

 Cuvier and called Hectocotylus octopodis. 



History.— The common cuttle fish of the Mediterranean have been 

 well-known animals for a veiy long period of time, and were called polypsi 

 by Aristotle and the Latin and medieval authors. The class Cephalopoda 



