38 



ESSENTIALS OF BIOLOGY 



the most typical worms describes their outer form. The body 

 is elongated, with typical triple-symmetry and it is often externally 

 " ringed " because of its division into the metameric, fore-and-aft 

 segments. The " worm "-group is far more heterogeneous than 

 any of the others we consider here ; nevertheless, a large fraction 

 of all the forms included in it have the essential structure displayed 

 by the earth-worm. 



The Molluscan Type. A vast number of animal 



lit. 



species belong to groups represented by the familiar forms, 

 Bivalves (oyster, for example) Univalves (whelk, or periwinkle, 

 or common snail, cuttlefish, etc. Examples are represented in 

 the figure. 



Foot 



'Heart 

 ■GUIs 

 ■MouniJbB' 

 iTztestine 



Foot 



4 



Fig. 8. 



I, A Bivalve (or Lamellibranch), the Cockle; 2, a Univalve (or Gasteropod) ; 3, another 

 Univalve, a marine slug, or Nudibranch ; 4, transverse section through a Bivalve, such as 

 the Cockle ; 5, a Squid (Cephalopod). 



Here we have an animal type that is homogeneous in spite of 

 much outer diversity of unessential nature. Typically, the 

 mollusc is a soft-bodied animal partially enclosed in a shell of 

 calcareous material. The symmetry is triple. Apparently very 

 many molluscan bodies are unisegmental, but the actual, or con- 

 cealed existence of the pluri-segmental body can be demonstrated. 

 There is a strongly muscular part of the body (the " foot," for 

 instance). The body is covered by a fleshy " mantle." The shell 

 can be closed (in many groups) so as completely to enclose the 

 body. The shell assumes most varied forms. 



11^. The Arthropod Type. Arthropods include the Crus- 



