THE SHAPES AND SIZES OF ANIMALS 149 



unusually great diversity of size. The majority of 

 arthropods are small, and a length of four inches is 

 a long way above the average, while many large 

 and very numerous groups such as Entomostraca 

 are minute, indeed almost microscopic, in their 

 dimensions. Crabs may reach a large size, but 

 this is almost entirely due to great elongation of 

 their legs. 



Amongst Mollusca a much larger size may be 

 attained. Of Lamellibranchs the genus Tridacna 

 affords the giants, shells of these huge clams having 

 been found measuring two feet across and weighing 

 upwards of five hundred pounds, the animal itself 

 exceeding twenty pounds in weight. Gastropods 

 also reach a large size, but are far exceeded by the 

 Cephalopods, or cuttle-fish, which group yields the 

 largest invertebrates known to exist. 



From time to time the bodies of enormous cuttle- 

 fish are cast up on the shores in various parts of 

 the world. Two or three very large specimens 

 have occurred on the West Coast of Ireland, others 

 are described from New Zealand, but by far the 

 largest and also the most numerous cases are from 

 the East Coast of North America, more especially 

 the Coast of Newfoundland. Some of the speci- 

 mens are of really gigantic proportions, a length 

 of body of as much as twenty feet having been 

 measured, while the long arms, or tentacles as 

 they are often named, were thirty or even forty 

 feet in length. 



The Gastropods yield some very instructive series 

 of forms, illustrating in a remarkable way the influ- 



