156 



MANUAL or THE MOLLUSCl. 



mandibles of birds. The tongue is large and fleshy ; part of its 

 surface is sentient, whilst the rest is armed with recurved spines ; 

 their eyes are large, and placed on the sides of the head. In all 

 probability they possess the faculty both of smelling and hearing. 

 All are carnivorous, and live in the sea. 



The nervous system is more concentrated than in the other 

 moUusca, and the brain is protected by a cartilage. The respira- 

 tory organs consist of two or four plume-like gills, placed 

 symmetrically on the sides of the body, in a large branchial 

 cavity, opening forwards on the under* side of the head : in the 

 middle of this opening is placed the siphon or funnel. The sexes 

 are always distinct. The cephalopoda are divided into two 

 orders, the names of which are derived from the number of the 

 hrancJiice. 



Oeder I. — ^DiBRAK-CHiATA, Owen. 



Animal swimming; naked. Head distinct. Eyes sessile, 

 prominent. Mandibles horny (PL I., fig. 2). Arms eight or 

 ten, provided with suckers. Body round or elongated, usually 

 with a pair of fins ; hranchice two, furnished with muscular 

 ventricles ; ink-gland always present ; funnel a complete tube. 



Shell internal (except in argonauta), horny or shelly, with or 

 without air-chambers. The shell of the argonaut does not 

 correspond with the ordinary shell of mollusks. (See p. 39.) 



The typical forms of the cuttle-fishes were well described by 

 Aristotle, and have been repeatedly examined by modem 

 naturalists ; yet, until Professor Owen demonstrated the exist- 

 ence of a second order of cephalopods, departing from all the 

 above-mentioned characters, it was not clearly understood how 

 inseparably the organisation of the cuttle-fishes was connected 

 with their condition as swimming mollusca, breathing by tivo 

 gills. There are two types of lung structure among the dibran- 

 chiates. Thus, in Octopus and Sepia the gills form a cylinder, 

 while in Loligo and other genera they form a half cylinder. 



The characters which co-exist with the two gills, are the 

 internal rudimentary shell, and the substitution of other means 

 of escape and defence, than those which an external shell would 

 have afforded ; viz. , powerful arms, furnished with suckers ; the 



* According to the established usage, we designate that the binder or ventral side of 

 the body, on which the funnel is placed. But if the cuttle fishes are compared with 

 the nucleobranchs, or the nautilus with the holostomatous gasteropoda, their external 

 analogies seem to favour an opposite conclusion. There are many terms in use which 

 are apt to mislead, such as fins, arms, &c. ; they have a definite meaning when applied 

 to the vertebrata, but not so when applied to the invertebrata. 



