TYPE MOLLUSOA. 



359 



rated from tlie rest of the lobe — tliat of the left side becoming 

 modified into a conical structure, lamellated at the extremity 

 and destitute of tentacles, forming what is termed the spadix, 

 probably homologous with the hectocotylized arm of the 

 male Octopods and Decapods. 



2. Order Dibranchia. 



The members of this order, which includes the majority 

 of living Cephalopods, possess but a single pair of ctenidia, 

 nephridia, and auricles, and lack 

 the direct communication of the 

 viscero-pericardial cavity with the 

 exterior as well as the osphradia 

 which occur in Nautilus. The 

 portion of the foot which is fused 

 with the head is drawn out into 

 a number of arms provided with 

 suckers, which seem to represent 

 the tentacles and their sheaths 

 found in Nautilus. The suckers 

 are very numerous and may be , 

 arranged in from one to four rows 

 on the ventral surface of the 

 arms, the margin of each sucker 

 being in some forms strengthened 

 by a horny ring, which may be 

 toothed. The number of the 

 arms varies, being either eight 

 or ten ; and, since, other struc- 

 tural differences are associated 

 with this difference, the order 

 may be divided into two suborders Fig. 160.— LoUgo pallida, Doesal 

 — the Octopoda with eight arms, ^^'^^ ^^^^"^ emerton from verrild. 

 including the genera Octopus, Tremoctopus, and Argonauta 

 (Fig. 158), and the Decapoda with ten arms, the genera Spi- 

 rula, Ommastrephes, Sepia, and Loligo (Fig. 160) belonging to 

 this group. 



In the Decapoda the ten arms are not of equal size, one 



