CRUSTACEA. 39 



suture which divides the two sternal plates may be ob- 

 served. On the abdomen the median suture cannot be 

 detected, but the lateral borders of the tergal plates are 

 well defined, though this is due rather to the lighter color 

 of these parts than to any other cause. 



While we can thus describe a typical ring, it must not 

 be forgotten that in the sterna of the thorax there is 

 more than one pair of plates. In fact, the number of 

 plates is, like other parts, a variable quantity, and in fol- 

 lowing the text-books this fact should be borne in mind. 



Comparing now the thoracic segments with the third 

 abdominal ring, the sterna and pleura are recognized, 

 while the terga appear to be wanting. This, however, 

 is denied by some, who consider that these rings have 

 simply opened along the middle of their tergal portions 

 and spread apart. This, of course, has brought the two 

 halves of each tergum on opposite sides. We can only 

 account for this splitting of the rings, if this occurred, and 

 their existing forms, by supposing that their dorsal por- 

 tions were not needed for the protection of the internal 

 organs, as in the abdominal rings, the carapace here being 

 quite sufficient for that purpose. 



The consolidation of the rings of the cephalothorax 

 is usually accounted for by the theory of cephalization 

 or head-making. This theory, originated by Professor 

 Dana of New Haven, assumes that in the higher forms 

 of Crustacea and other types there is a concentration 

 of parts and organs towards the head, and a corre- 

 sponding series of modifications introduced, enabling 

 these to subserve the purposes of the head better, and 

 dividing them from the walking-limbs, etc. Yet this 

 theory does not seem to offer an adequate explanation 

 for all the facts observed. In many Crustacea, Limulus 

 (Fig. 31), etc., the cephalic and thoracic appendages 

 have become concentrated around the mouth. 



