EXPLANATION OF THE TERMS USED. 



Cephalopoda (fig. 7). 



THE Cephalopoda are all marine. The body is symmetrical ; the head is distinct, with 

 a pair of large eyes, and the mouth in the centre. Inside the mouth are a pair of 

 horny mandibles, shaped like the beak of a parrot. The mouth is surrounded by a 

 circle of arms. In the Octopod cuttle fish there are eight of these arms, all being 

 nearly alike, and provided with rows of suckers (fig 71) shaped like small cups. In 

 the Decapod cuttle fish there are ten arms, but two of these, called TENTACULAB 

 AR.VTS (fig. 7), are longer than the others, and bear suckers only at their extremities, 

 which are sometimes enlarged or club-shaped. The respiratory organs consist of 

 two or four gills on the sides of the body, in a large branchial cavity opening forward 

 on the under side of the head ; in the middle of this opening is placed the SIPHON 

 or FUNNEL. Water is admitted into the branchiae by the expansion of the outer 

 mantle ; when the mantle is contracted, this water is forcibly expelled through the 

 siphon, forcing the animal in a backward direction. The siphon is provided with a 

 valve, which permits the egress of water, but prevents its ingress. 



All the Cephalopoda, except the Nautilus and the Argonaut, are naked, but are 

 provided with an internal shell known as the cuttle-bone or PEN, which is either 

 horny or calcareous. In some cases a chambered portion, or PHRAGMACONE, is 

 added ; and in Spirula the phragrnacone forms the whole of the internal shell, and is 

 coiled into a spiral, the coils of which are not in contact. The shell of the Argo- 

 naut, or Paper Nautilus, is not divided into chambers, and the animal is not attached 

 to it, but can leave it at pleasure. The female only is provided with this shell, and 

 she uses the empty apex as a receptacle for the ova. 



Pteropoda. 



The Pteropoda are all oceanic, swimming by two wing-like fins on each side of 

 the head. They are usually enclosed in a horny translucent shell, which is sym- 

 metrical. They form the principal food of the whale in high latitudes, but are 

 seldom seen by the shore naturalist. 



Heteropoda. 



This class also consists entirely of oceanic animals, and the purple snail-like shell 

 of lanthina is all that the shore naturalist is likely to collect. In them the foot of 

 the Gasteropoda is modified into a raft-like float, while motion is effected by means 

 of a fin-like tail. 



Gasteropoda (fig. 8). 



The Gasteropoda, or Univalves, can be divided into two groups, one breathing air 

 (PTTLHONIFERA), the other water (BEANCHIFEEA). The young of the former when 



