ORGANIC EVOLUTION 



233 



to interpretation of homologies in any appendage are likely 

 to be found in other adjacent 

 appendages, which , because of prox- 

 imity, have been subject to some- 

 what similar influences. 



Study 31. Observations on plas- 

 ticity of form and persistence 

 of type in Malacostraca. 



Materials needed : Specimens 

 preserved in formalin of represen- 

 tative of at least three orders of 

 Malacostraca, Cambarus (fig. 143), 

 Asellus (fig. 144), and Gammarus 

 (fig. 145): if such marine forms 

 as Mysis (fig. 1580; and Squilla 

 and any of the crabs are available, 

 all the better. Also, a few females 

 of each type, bearing eggs, and 

 a few live specimens for use in de- 

 termining the functions of the 

 appendages. Also, slide mounts 

 of such appendages as are too 

 small to be readily examined in 

 place, or easily removed. 



Observe the living specimens, 

 noting especially the different uses 

 to which the appendages are put in 

 locomotion. 



Demonstrate the very special 

 water-propelling function of the 

 ''gill scoop" that is appended to the 

 outside of the second maxilla in the 

 crawfish, by holding the point of a 

 copying pencil in the water beside 



FIG. 144. Asellus aquaticus 

 (after Sars). a, dorsal view; 

 b, ventral view of abdomen 

 of female ; x, last segment 

 of thorax ; y, appendage of 

 abbreviated first abdomi- 

 nal segment (the second 

 segment is without appen- 

 dages in the female) ; z, gill 

 cover (operculum). 



