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MANUAL OF ZOOLOGY. 



SECTION B. DECAPODA. The Cephalopods of this section 

 have eight arms and two additional 

 " tentacles," which are much longer 

 than the true arms, and have ex- 

 panded club-shaped extremities (fig. 

 107). The suckers are pedunculated; 

 the body is always provided with 

 lateral fins, and the shell is always 

 internal. 



This section comprises the three 

 living families of the Tatthidce, Sepi- 

 adcz, and the Spirnlidce, and the extinct 

 family of the Belemnitidce. 



The family of the Ttuthidce com- 

 prises the Calamaries or Squids, cha- 

 racterised by the possession of an 

 elongated body with lateral fins. 

 The shell is internal and horny, con- 

 sisting of a median shaft and of two 

 lateral wings ; it is termed the "gladi- 

 us" or "pen," and in old specimens 

 several may be found lodged in the 

 mantle, one behind the other. In 

 the common Calamary (Loligo) the 

 fourth left arm of the male is meta- 

 morphosed towards its extremity to 

 subserve reproduction. 



In the family of the Sepiadcz the 

 internal shell is calcareous ("cuttle- 

 bone " or " sepiostaire")and is in the 

 fonn of a broad plate, having an im- 

 perfectly chambered apex. In the 

 living members of the family the body 

 is provided with long lateral fins, 

 sometimes as long and as wide as the 

 body itself. 



In the singular family of the Spiru- 

 lidce the internal skeleton is in the 

 form of a nacreous, discoidal shell, 

 the whorls of which are not in con- 

 tact with one another, and which is 

 divided into a series of chambers by means of partitions or 

 septa which are pierced by a ventral tube or " siphuncle." 

 The body is provided with minute terminal fins. The shell of 

 the Spirula commonly known as the " post-horn " is similar 



Fig. no. Diagram of Belem- 

 nite (after Professor Phillips). 

 r Horny pen or "pro-ostra- 

 cum ;" p Chambered " phrag- 

 macone" in its cavity (a) or 

 ' ' alveolus ; " g" Guard. " 



