MOLLUSCA PAID 
(1.) CHARACTERISTICS.— The Tetrabranchiata are characterised as 
follows. The siphon is not complete ; the edges overlap one 
another, but 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, but the left is in both cases 
rudimentary. The shell is large, external, and chambered. 
No ink sac, salivary glands, or branchial hearts exist. 
This order contains very many extinct forms, but only 
one living genus—Nautilus. 
(i.) CHARACTERISTICS.—The Dibranchiata have but 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 bear cwp-like 
suckers. Branchial hearts exist at the base of the gills. The 
coclom communicates with the exterior through the nephridia, 
and not directly ; the oviducts may be 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 Decapoda 
and the Octopoda. 
a. CHARACTERISTICS.— The Decapoda have ten arnvs, two of which 
are very long, and differ in appearance from the others. The 
suckers are stalked, and provided with a horny ring. The 
body is elongated, and bears lateral fins. The shell is enclosed 
by an upgrowth of the mantle, and is therefore internal. 
B. CHARACTERISTICS.—The Octopoda have eight similar arms, 
bearing sessile suckers, which are not strengthened by a horny 
ring. The body ws short and globular. 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 
egos 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 
