CEPHALOPODA. 341 



brane^ like the mantle in the testaceous mollusks ; by means of these 

 membranes the animal, in fact, forms for itself an extremely light, 

 slightly flexible, and elastic, but calcareous, symmetrical shell, which 

 is simple, and not divided into chambers ; the vacated portion commu- 

 nicating with the rest, and being used by the inhabitant as the recep- 

 tacle for the eggs. The siphon is without a valve, but is articulated 

 at its base on each side to the inner surface of the mantle. The second 

 family of the Octopods I have termed Nuda, the species not being 

 provided with an external shell. The first pair of arms is elongated, 

 and contracts to a point : the funnel or siphon is without an internal 

 valve or external joints. The rudimental shell is represented by two 

 short styles, encysted in the substance of the mantle. The typical 

 genus of this family is termed Octopus, in which the arms are pro- 

 vided with a double alternate series of sessile acetabula. In a second 

 genus Eledone, the arms are provided with a single series of a(;etabula. 

 In the Cirroteuthis a pair of filaments project between each of the 

 suckers. 



The shell or dermal skeleton, which has been progressively reduced 

 in the present highly organised class, attains its lowest or most rudi- 

 mental condition in the Octopus and Eledone. The genus in which 

 the shell most nearly resembles that of the tetrabranchiate Cephalopods, 

 belongs to the Spirula. A few mutilated specimens which had 

 reached this country during this present century, had demonstrated it 

 to be an internal shell, and the more perfect specimen dissected by 

 M. de Blainville in 1839, proved it to have the characteristic organi- 

 sation of theDibranchiate order, and to possess, as Peron had indicated, 

 the eight short arms and the two long tentacula of the Decapodous 

 tribe. The shell of the Spirula {Jig. 134.) is perfectly symmetrical, 

 convoluted in a vertical plane with the whorls con- 

 tiguous, but not touching. The shell commences 

 by a small oval cell, followed by a series of cham- 

 bers (6), which rapidly increase in size. The septa 

 («) are concave towards the outlet, and are per- 

 spiruiaaustiaiis. foratcd by a siphon at the internal or concave 

 margin of the shell. A small funnel-shaped tube (c) is continued 

 backwards from each perforation ; and its apex penetrates the mouth of 

 the succeeding tube. The circumference of the siphonic aperture is 

 impressed, as Mr. Stokes first observed, by a 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. 



z 3 



