342 LECTURE XXIV. 



The principal characters of the internal shell of the cuttle-fish have 

 already been pointed out in the illustration of its analogies with that of 

 the Belemnite. The preparations, Nos. 106, 107, 108, demonstrate the 

 great proportion of animal membrane which enters into the compo- 

 sition of this light friable laminated shell. 



In the rest of the Decapoda, the rudimental shell consists exclusively 

 of animal matter, of the consistency of horn, and presents either the 

 form of a pen, as in the Calamary, or that of a straight three-edged 

 sword, as in the Onychoteuthis. This body was called ' xiphos' by 

 Aristotle, and * gladius ' by Pliny. In the Sepioteuthis it is as broad 

 in proportion to its length as the cuttle bone. In the Onychoteuthis^ 

 LoUgo, and Loligopsis, it is much narrower, but is as long as the mantle. 

 In Sepiola and Rossia, the gladius, commencing at the anterior 

 margin of the mantle, ends before it has reached half way down the 

 back. In the Octopus and Eledone, the last traces of a shell exist in 

 the form of two small amber- coloured styliform bodies, contained 

 loosely in capsules in the substance of the mantle. 



The skin of the naked Cephalopod is generally thin and lubricous, 

 and can be more easily detached from the subjacent muscles than 

 in the inferior Mollusks. In some of the smaller Cephalopods it is 

 semitransparent ; it is densest in the Calamaries, in which the 

 epidermal system is most developed, as is exemplified in the horny 

 rings or hooks upon the acetabula. In the Octopods the epidermis is 

 reflected over the interior of the acetabula without being condensed 

 into horn. Upon the body the epiderm may generally be detached in 

 the form of a thick white elastic semitransparent layer. The second, 

 or pigmental layer of the skin, analogous to the rete mucosum, con- 

 sists of numerous cells of a flattened oval or circular form, containing 

 coloured particles suspended in a fluid. The colour is rarely the 

 same in all the cells ; the most constant kind generally corresponds 

 more or less closely with the tint of the inky secretion. In the Sepia 

 there is a second series of vesicles containing a deep yellow or 

 brownish pigment : in the Loligo vidgaris there are three kinds of 

 coloured vesicles, yellow, rose-red, and brown : in the Octopus vid- 

 garis there are four kinds of vesicles, red, yellow, blue, and black. 

 In the skin of the Argonauta all the colours which have been ob- 

 served in other Cephalopods are present, and contained in their ap- 

 propriate cells. These cells possess the power of rapid alternate 

 contractions and expansions, by which the pigment can be driven into 

 the deeper parts of the corium or brought into contact Mith the 

 semitransparent epiderm. If the skin of an Octopus be slightly 

 touched, the colour will be accumulated, gradually or rapidly, like a 

 cloud or a blush upon the irritated surface. If a portion of the skin be 



