MOLLUSC A 211 



(i.) Characteristics. — The, TetrahraTwhiata are characterised as 

 follows. The siphon is not complete ; the edges overlap one 

 another, hut are not fused; the lobes of the circumoral foot 

 carry tentacles, not suckers. There are two pairs of ctenidia 

 and two pairs of nephridia. The coelom opens straight on 

 to the exterior, and not into the nephridia. There are two 

 oviducts and two vasa deferentia, hut the left is in hoth cases 

 rudimentary. The shell is large, external, and chamhered. 

 No ink sac, salivary glands, or hranchial hearts exist. 

 This order contains very many extinct forms, but only 

 one living genus — Nautilus. 



(ii.) Characteristics. — The Bihranohiata have hut one pair of 

 ctenidia and one pair of nephridia. The edges of the siphon 

 have fused so as to form a complete funnel. The arms or 

 processes of the foot which surround the head hear cup-like 

 suckers. Branchial hearts exist cut the hose of the gills. The 

 coelom communicates with the exterior through the nephridia, 

 and not directly ; the oviducts may he paired or single ; the 

 vas deferens, with one exception, is single. An ink sac and 

 salivary glands exist. The sense organs are highly developed. 

 The Dibranchiata comprise two sub-orders : the Decappda 

 and the Octopoda. 



a. Characteristics. — The Becapoda have ten arms, two of which 

 are very long, and differ in appearance from the others. The 

 suckers are stalked, and provided with a horny ring. The 

 hody is elongated, and hears lateral fins. The shell is enclosed 

 hy an upgrowth of the mantle, and is therefore internal. 

 ^. Characteristics. — The Octopoda have eight similar arms, 

 hearing sessile suckers, which are not strengthened hy a horny 

 ring. The hody is short and glohular. The oviducts are 

 paired. There is no shell in or on the visceral hump. 

 The Cuttle-fish or Sepia officinalis is common in most seas, 

 and in the spring, when it approaches the shore to deposit its 

 eggs amongst the rocks, it is easily caught. In considering the 

 anatomy of this form, it is important to orientate the animal 

 correctly; with this view it should be placed mouth down- 

 wards, then its foot will be ventral, its visceral hump dorsal, 

 and its mantle cavity posterior. For the sake of convenience 

 it is, however, better to twist the animal through a right 



