302 INTRODUCTORY NOTE TO THE CRUSTACEA. 



as do also other characteristics ; the loosening of the outer maxillipeds ia 

 the true Cancers, and most swimming Crustacea and Corystoids, is another 

 step in this relaxation ; the elongation of the antennae in the Corystoida 

 and Anomoura is another step ; the loosening of the abdomen from the 

 ventral surface of the cephalothorax ; its becoming loosely inflexed or 

 even extended ; its taking appendages — are among the other steps seen in 

 the Anomoura ; the outer maxillipeds becoming pediform, and then the 

 next pair pediform also, showing a tendency to a passage from the mouth- 

 peries to the foot-series, are other steps downward, observed in the Macrura ; 

 and the elongated abdomen with its regular series of organs as well as the 

 elongated antennae, the union without fossettes, and eyes without sockets, 

 all exhibit the relaxation of centralization that marks the Macrura. 



A further degradation is seen in the obsolescence of some of the pairs of 

 feet and abdominal appendages, as in the Mysis group ; and the same 

 principle is exemplified in the Brachyura, where the posterior cephalo- 

 thoracic legs become small or rudimentary, or swimming legs. 



There are, hence, two methods by which the passage of Crustacea from 

 the higher to the lower grades takes place : 



1. A diminution of the centralization leading to an enlargement of the 

 circumference or sphere of growth at the expense of concentration, as in the 

 eloncation of the anteimae, a transfer of the maxillipeds to the foot-series, 

 and the elongation of the abdomen and abdominal appendages. 



2. A diminution of force as compared with the size of the structure, 

 leadinof to an abbreviation or obsolescence of some of the circumferential 



o 



organs, as the posterior or cephalothoracic legs, or anterior antennae, or 

 the abdominal appendages (if such appendages belong to the type embrac- 

 ing the species). 



The Macrura, Anomoura, and Brachyura are alike in having normally" 

 nine cephalic annul! (out of the fourteen cephalothoracic), and but five 

 foot-annuli. The Mysis and Squilla groups are in the same category. 

 There are species that show a tendency to a transfer of the posterior 

 mouth-annuli or appendages to the foot-series, but it is only a tendency. 

 These together constitute the First type among Crustacea. 



In the Seco7id type, there are seven cephalic annuli and pairs of append- 

 ages, and seven foot-annuli or pairs of feet ; such are the Isopoda, Aniso- 

 poda and Amphipoda. 



In the Third type, there are normally six (or five) cephalic annuli, out 

 of the whole normal number, fourteen, — the eight (or nine) posterior annuli 

 belonging to the foot-series, part of which (the three posterior pairs and 

 often more) are usually obsolete. Moreover, the abdomen, by the second 

 law of degradation, mentioned above, is without appendages — such are 

 the Entomostraca. 



