GENERAL NOTES ON CRUSTACEANS. 295 



but it is difficult to say anything definite as to the more remote 

 ancestry. 



We naturally think of a segmented worm-type as a plausible starting- 

 point for Crustaceans, and it is not difficult to understand how a 

 development of cuticular chitin would tend to produce a flexibly jointed 

 limb out of an unjointed parapodium ; how the mouth might be shunted 



FIG. 142. Schizopod (Mysis JlexuosA), from side. 



., Brood-pouch borne on posterior thoracic limbs ; o. , otocyst 

 in tail. Note eight pairs of similar biramose thoracic feet. 

 The last two thoracic segments are not covered by the 

 shield. 



a little backwards, and two appendages and ganglia a little forwards ; 

 and how division of labour would result in the differentiation of distinct 

 regions. 



GENERAL NOTES ON CRUSTACEANS. 



Of a class that includes animals so diverse as crabs, 

 lobsters, shrimps, " beach-fleas," " wood-lice," barnacles, 

 acorn-shells, and " water-fleas," it is difficult to state general 

 characteristics, other than those facts of structure which 

 we have already summarised. 



Admitting the parasitism of many Crustaceans, and the 

 sedentary life of barnacles and acorn-bhells, we must still 

 allow that great activity characterises the class. With this 

 may be connected the brilliant colouring, the power of 

 colour change, and the phosphorescence of many forms. 



Except in the case of a few primitive and degenerate 

 forms, the Crustacea are all segmented. In this, in the 

 presence of hollow jointed appendages, in the reduction of 

 the coelom, and in their firm chitinous cuticle, the Crustacea 



