THE FRESH-WATER CRAYFISH {Astacus) 



THE GAPE CRAWFISH {Jasus lalandii) 



First Day 



External Features. 



1. Make a drawing of the animal viewed from the dorsal surface to show : 

 (a) The large carapace or cephalo-thoracic shield covering the whole of the 

 head region and thorax. The carapace is marked off by grooves into 

 definite areas, which may be named after the organs in the body over 

 which they lie. There is a short transverse groove in the middle of 

 the carapace called the cervical or neck groove, which marks oft the 

 head region from the thorax. The part of the carapace, in front of 

 this groove covers the stomach, and may therefore be called (i) the 

 cephalo-gastric region. The part just behind this covers the heart, 

 and may be called (ii) the cardiac or thoracic region. On each side of 

 the body there is a part of the carapace which protects the gills ; these 

 gill-covers are called the branchiostegites, and form (iii) the branchial 

 region of the carapace. The grooves which mark them off from the 

 cardiac region are called the branchio -cardiac grooves. If these 

 grooves are traced forwards it will be seen that each divides into two 

 branches, one' passing forward towards the base of the antenna and 

 the other and more distinct one passing to the underside of the body. 

 These two branches enclose a small area of the carapace covering the 

 liver and which is therefore called (iv) the hepatic region. 



On the cephalo-gastric region a prominent projection of the carapace 

 in the middle line is seen ; this is the rostrum. It has a broad base 

 and ends in front in a sharp spine. It has a smaller spine on each 

 side. \In Jasus the rostrum is small.] 

 {b) The six abdominal segments, at the end of which is the telson which 

 may represent the 7th abdominal segment. Each has a ring-like 

 form, presenting a broad dorsal region or tergum, a narrow ventral 

 region or sternum, and downwardly directed lateral processes, the 

 pleura. 



(c) The stalked eyes, situated at the anterior end of the cephalo-thorax, one 



on either side of the rostrum. 



(d) The following appendages as seen from above : — 



(i) The pair of biramous antennules, each with an otocyst, the 

 auditory and balancing organ, on their basal joints. [In 

 Jasus the antennules have each a pair of small terminal flagella.] 



(ii) The pair of antennae. ' 



(iii) The five pairs of walking legs. The anterior three walking legs 

 of each side end in a pincer-like claw. The first walking leg 

 (the chehped) is much stouter than any of the others^ its claw 



