MARINE MOLLUSCS 



191 



molluscs; and no internal skeleton of any kind is to be found 

 except in the division containing the Cuttlefishes, the ' bone ' of 

 which is one of the common objects washed up on our shores by 

 the breakers. 



In all the molluscs there is a well-formed digestive tube, and 

 often a complex arrangement of small teeth which sever the food 

 by a rasp-like action. There is also a well-formed heart, consist- 

 ing of two or more cavities, by means of which the blood is forced 

 through the body ; but, as a rule, bloodvessels are either few or 

 absent, the blood being driven through spaces between the tissues 

 that serve the same purpose. 



Fro. 126. LARVAE OF MOLLUSCS 

 , ciliated ' velum ' ; /, rudimen tal foot 



The nervous system consists of a few masses of nerve substance 

 (ganglia), connected by nerve cords, and sending off fibres to 

 various parts of the body, the principal ganglion being one 

 situated close to the mouth, and often surrounding the first portion 

 of the digestive tube. 



The animals of this sub-kingdom are grouped into three principal 

 and well-marked divisions the Lamellibranchs, or Plate-gilled 

 molluscs, the gills of which are composed of plate-like layers, and 

 the headless bodies enclosed in a bivalve shell ; the Cephalophora, 

 or head-bearing molluscs, protected by a univalve shell ; and the 

 Cephalopoda, or Head-footed molluscs, so called because the mouth 

 is surrounded by tentacles or arms by which the animal can cling 

 to objects or seize its prey. 



We shall deal with these three divisions in the above order, 



