406 



CHARACTERS OF INVERTEBRATE ANIMALS 



compare, e.g., the antennae, great pincers, and swimmerets ; 2, 

 variation in relative size: compare outer and inner branches in 

 an average swimmeret and the side-pieces of the tail-fin; 3. 

 variation in number: taking the Y- shape as the generalized 

 type, the gill-plate is an addition in some cases, while in others 

 the outer branch is much reduced or even absent; 4. f^ls^on 

 of parts: seen in the first abdominal appendages of the male. 



This is a convenient point at which to compare some of 

 the anterior appendages of Lobster with those of air-breathing 

 Arthropods. One view is represented by the following table : 



It should be mentioned that many difficulties attend the 

 comparison of segments and appendages in different groups, for 

 one can seldom be quite certain that segments really correspond 

 in different cases, and, even if they did, some appendages are 

 apt to shift their position in the course of development. 



The Lobster is protected by a firm shelly exoskeleton in the 

 form of a horny layer which is thin where mobility is required, 

 while elsewhere it is thick and hardened by the deposit of salts 

 of lime. This strong suit of armour cannot increase in size as 

 the animal grows, and the exigencies of the case are met by a 

 process of moulting, which takes place frequently in young 

 animals and at longer intervals later on. At the commence- 

 ment of the operation a transverse split appears along the back, 

 where the carapace joins the tail, and through this opening the 

 animal painfully makes its way out. A sheltered corner is 

 chosen for moulting purposes, and here the soft and defenceless 

 creature remains till its new armour is properly developed and 

 hardened. 



The chief external characters have now been reviewed, and 



