'.[Proc. Eor. Soc. Victoria, 31 (N.S.), Part II., 1919]. 



Akt. Xf. — Ahnornial Development of the Head Appendages 

 in the Crayfish, Parachaeraps hlcarivatvs Gray, 



By JANET W. RAFF, M.Sc. 

 (Demonstrator in Biology in the University of Melloiirne). 



(With Plate XV.). 



[Read 12th September, 1918]. 



The specimen described in the following notes is a small male 

 form of Parachaeraps hicarinatus — the common yabbie of our pools, 

 and streams. It measures 3J inches in length, and it shows certaia 

 abnormalities regarding the positions of the head appendages. 

 The .specimen was handed to me for examination by Dr. Sweet,, 

 who thought the irregularities worthy of note. 



When viewed from W\q dorsal surface the abnormalities consist 

 of the following points, as shown in Fig. 1 : — 



(1) Tlie right eye appears slightly larger, and longer stalked than 

 the left. 



(2) The antennides are pushed out of position so that both are- 

 seen to the right side of the rostrum, the left one being situated 

 above the right. 



(.3) The exopodites or scaphocerites of the anfennae, when in- 

 clined inwaids, are seen to be on a level with the left antennule, and 

 lie one on each side of it. The right one extends slightly more- 

 anteriorly than the left. The endopodites of the antennae are on 

 a level witli the right antennule. 



Tlie head region appears distinctly broader when viewed dorsally 

 than in normal specimens of the same size. 



When examined from the ventred surface the abnormalities are 

 mucli moie marked (Fig. 2). The most striking point of irregu- 

 larity lies in the abnormal position of the mandibles^ the right one 

 being situated at a higher level, i.e., more anteriorly, than the left, 

 it ])einLr tlius impossible for the two mandibles to bite against one 

 another. The mavfiihle of the right side is placed on a level with- 

 the lahnim, instead of posterior to it, and its " teeth " rest against 

 roughnesses on the inner edge of the labrum, which latter is situated 

 entirely on the left side of the head, and not in the middle line. 



The sternal portion in front of the mouth (eplsfonia) presents a 

 very abnormal appearance. It is practically undeveloped on the 



