CEPHALOPODA. 607 



circle of minute pits. The shell seems to be exclusively composed of 

 a fine white nacreous substance ; it is imbedded in the posterior part 

 of the mantle, with a small part of its surface exposed on both the 

 upper and lower sides of the animal's body : the exposed parts of the 

 shell are invested by a granular straw-coloured epidermis. The 

 shelly siphon is traversed by a tubular membranous siphon. 



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 com- 

 position of this slight friable laminated shell. The laminae consist 

 of a subopake minutely granular substance, and are connected 

 together by thin wavy leaves of a transparent substance, the folds of 

 which resemble little pillars, and are marked by fine transverse striae, 

 like those on the prisms of the shell of the Pinna. 



In the rest of the Decapoda the rudimental shell consists exclusively 

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

 form of a pen, as in the common Calamary, or that of a straight 

 three-edged sword, as in the hooked squids.* This body was called 

 " xiphos " by Aristotle, and " gladius " by Pliny. In Sepioteuthis it is 

 as broad in proportion to its length as the cuttle-bone. In Onycho- 

 teuthis, Loligo, 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. All 

 these bodies present a minutely laminated structure when trans- 

 versely bisected. The shell of the Argonaut presents a minutely 

 granular texture ; "it exhibits no trace of cellular or prismatic tissue."! 



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

 and can be more easily detached from the subjacent muscles than 

 in the inferior MoUusks. 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 tlie 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- 



* In some extinct Cephalopods from oolitic strata the shell was externally 

 calcareous, internally homy, e. g. Coccoteuthis. See CCLXXII. 

 t CCXXXI 



