606 LECTURE XXIV. 



tremity into a broad tliiu membrane, like the mantle in the testaceous 

 moUusks ; by means of tlie^e membranes, the animal, in fact, forms 

 for itself an extremely light, slightly flexible, and elastic, but calca- 

 reous, symmetrical shell, which is simple, and not divided into 

 chambers ; the vacated portion communicating with the rest, and 

 being used by the inhabitant as tlie receptacle for the eggs : it has 

 no muscular attachment to the body. * 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 is termed ^^itda, 

 the species not being provided with an external shell. The first pair 

 of arras 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 provided with a double alternate series of sessile aceta- 

 bula. In a second genus Eledone, the arms are provided with a 

 single series of acetabula. The third family is the Pinnata, typified 

 by the Sciadephorus, which has a pair of filaments between each of 

 the suckers, and a pair of fins from the sides of the body.f 



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 Cepha- 

 lopods, belongs to the Spirula. A few mutilated specimens had 

 demonstrated it to be an internal shell, and the more perfect examples 

 of the animal, dissected by M. de Blainville;!: and myself §, proved it to 

 have the characteristic organisation of the Dibrauchiate order, and 

 to possess, as Perou had indicated, the eight short arms and the two 

 lono- tentacula of the Decapodous tribe. The shell of the Spirula 

 (Jiff, 221.) is perfectly symmetrical, convoluted in a vertical plane 

 221 with the w^horls contiguous, but not touching. The 



shell commences by a small oval cell, followed by a 

 series of chambers {b), which rapidly increase in size. 

 The septa (a) are concave towards the outlet, and 

 are perforated by a siphon at the internal or concave 

 spiruinaustraiis. 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 



* The honour of having experimentally determined the true relation of the 

 Argonaut shell to its cephalopodic tenant, and the true functions of the expanded 

 arms, is due to Madame Jeanette Power, CCLXXIII. 



t CCCXCIX. & CCCC. t CCCCI. p. 396. 



§ CCCCJI. pi. iv. 



