Characteristics of Cephalopoda. Ixxxix 



collar and tlieir organs of special sense; the Tetrahranchiaia, in 

 which the internal skeleton attains much less importance than it 

 does in the BihrancJiiata, have an external shell; and the Dihran- 

 chiafa ordinarily possess an internal calcareous shell, as in Sejna, 

 Belemnitidae, and Spirula, or a rig-id support of conchiolin, as in Lo- 

 liginidae. The Octopodidae, however, in which locomotion ordinarily 

 is of the crawling kind, are devoid, with the exception of Cirrhoteu- 

 this, of any internal shell distinct from their various internal car- 

 tilaginous supports. The external shell of the female Argonautidae 

 is secreted by the extei^nal surface of the expanded ends of their two 

 mesially-placed dorsal arms ; the body of the animal is not attached 

 to it by the insertion of any muscles, and it is not homologous 

 with the external shell of the Nautilus, nor indeed with that of any 

 other Mollusc. The tegumentary system is distinguished, except in 

 the Tetrahrcmchiata, by the absence of cilia, and by the presence of 

 chromatophores, and of certain more deeply-placed lamellar cells 

 upon which their well-known power of changing colour depends. 

 The organs of animal life being all highly developed, those of 

 digestion, circulation, and respiration are so also in subservience 

 to them. The entrance to the digestive tract, besides being armed 

 with a rasping tongue, is further furnished with a powerful ex- 

 ternally-placed beak resembling that of a parrot, but having its 

 posterior segment the lai'ger of the two. 



Though the animals are exclusively carnivorous and marine, they 

 have always, with the exception of the TetrahrancJdata, a very largely 

 developed salivary system. A crop, and also a spiral stomachal 

 coecum, are usually present ; as also glandular appendages, which, 

 as being distinct, at least to the naked eye, from the great mass of 

 the liver, have been regarded as pancreatic. The intestine proper 

 does not describe any complex convolutions in its course to the anus, 

 which opens always in the middle line of the mantle cavity, and 

 contributes thus, with other arrangements, to give these animals 

 their very obvious appearance of bilateral symmetry. 



The systemic heart consists of a single ventricle, the walls of 

 which, in the higher Cephalopoda at least, are composed of trans- 

 versely-striated muscular tissue. The branchial veins which return 

 the aerated blood to the heart have, in some species of those orders, 

 dilatations developed upon them representing auricles : and in addi- 

 tion to the systemic heart, we find in all the Blhranchiata accessory 



