646 



MOLL USC A CLASS V. CEPHALOPODA. 



To the Tetrabranchiata probably also belongs the fossil group Ammonea, 

 in which the shell was external and of various shapes, but retained the 

 protoconch. 



ORDER II. DIBRANCHIATA 



Characterised by the possession of a single pair of gills. The funnel is 

 complete. The eye is highly developed, having both lens and cornea. The 

 shell, when present, is internal. 



Sub-order 1. DECAPODA. The members of this sub-order have ten arms. 

 Eight of these, often shorter than the body, are tapering and armed with 

 rows of suckers. Each sucker, or a^cetabulum^ is on a short stalk or pedicle, 

 and strengthened by a horny rim which is frequently toothed. The horny 

 rim is sometimes replaced by a hook which is retractile like the claw of a cat. 

 The other two arms, called the "prehensile tentacles " or ''tentacular arms," 

 placed one on either side, are longer than the rest and cylindrical in form, 

 with expanded ends armed with suckers, or with hooks. Just beneath the 

 expanded tips there is sometimes an arrangement of suckers enabling the 

 two to lock together and act in concert. These arms are retractile, and in 

 the cuttle-fish can be withdrawn into special pouches. 



The shell is concealed beneath the mantle on the side of the body opposite 

 to the pallial cavity. It is generally more or less rudimentary, but is most 

 developed in the cuttle-fish (Sepia) in which the "cuttle-bone" or "sepion" 

 occupies the whole length and nearly the width of the body. In another 

 form, Loliyo, the shell is reduced to a horny "pen" or "gladius." Spirilla 

 (Fig. 24), however, has a spirally- coiled chambered shell in 

 which the whorls are widely separated. This shell lies in the 

 hinder extremity of the body, and, in the few specimens of the 

 animal that have been obtained, is not completely concealed by 

 the mantle, though whether this is the natural state or due to 

 injury is not yet clearly ascertained. It is thus strikingly different 

 from the shells of other living Decapoda ; the gap between them 

 is, however, supplied in fossil forms. In some of these, the 

 Belemnites, the apex of the shell is enclosed in a calcareous 

 sheath called the guard, or by quarrymen "thunderbolts.'' The 

 shell of Spirula is common enough and frequently cast up on 

 our shores, but the animal is an inhabitant of deep water and 

 rarely obtained. The other members of the group frequent the 

 open sea, and often have lateral expansions of the mantle that 

 act as fins. 



Fig. 24. The Decapoda are divided into two sections : OIGOPSIDA, in 



SPIRULA. which the sea-water has access to the space between the cornea 

 of the^heli an( * lens of the eye, and MYOPSIDA, in which the cornea is 

 is indicated entire and water is not admitted. 



through ^^ e Bowing ar e the principal families: A, OIGOPSIDA 



the skin. Families Thysanoteuthidae, Ommastrephidae, Onychoteuthidae, 



Gonatidae, Chiroteuthidae, Cranchiadae, Spirulidae, and the fossil 



Decapods with "guarded" shells. B, MYOPSIDA Families Sepiolidse, 



Sepiadariidae, Idiosepiidae, Sepiidee, and Loliginidse. 



Sub-order 2. OCTOPODA. The members of this sub-order have short bag- 

 like bodies without tentacular arms, only the eight tapering ones, which are 



