ABDOMINAL APPENDAGES. , 137 



and of appendages : it is divided transversely by 

 an imperfect hinge, and its ventral surface, which 

 is only slightly calcified, is perforated by the 

 longitudinal slit-like anus. 



C. The Appendages. 



These are all reducible to a common type composed of the 

 three parts protopodite, exopodite, and endopodite, already 

 seen in the appendage of the typical segment, together with 

 an epipodite, which may be a simple plate or may bear 

 gill plumes. From such a typical form all the appendages 

 of the crayfish can be derived by variations in form and 

 relative size of the various parts, or by total suppression of 

 one or more parts. 



In the following description the appendages are numbered 

 from before backwards, and are distinguished by the large 

 Eoman numerals I. to XIX. 



Bemove the appendages of the right side one by one, 

 beginning with the hindmost, and taking special care to 

 remove each appendage entire. The characters of the smaller 

 ones can be best made out by examination in a drop of ivater, 

 between two slips of glass. 



1. The abdominal appendages, excepting the hindmost, are 

 of small size, and serve in the female to carry the 

 eggs. The last or twentieth segment has no appen- 

 dages. 



XIX. The sixth abdominal appendages are broad and 

 lamellar, and form with the telson the powerful 

 tail-fin. 



i. The protopodite is "short, broad, and undivided. 



ii. The endopodite is a flat oval plate, fringed at its 

 free edge with sette. 



iii. The exopodite is similar, but larger, and divided 

 by a transverse hinge into two parts. 



