THE YOUNG OF THE CRAYFISHES ASTACUS AND CAMBAKUS 35 



the figure since they are foi'eshortened there ; they do not stand ont in straight 

 continuation of the plane of the telson and sixth appendage but are curved 

 downward so that their tips tend to point forward under tlie animal. The 

 entire fan is thus somewhat concave on the ventral side and in life the larva 

 carries its abdomen with its tip close to the substi'atuni ui)on which it walks or 

 stands (fig. 41), and seen from above the fan is foreshortened. Both the habit 

 of carrying the fan inclined downward and the ciirved growth of the plumose 

 set* combine to make the fan a hollow scoop, which form would seem to be a 

 more efficient one for striking the water forwards and thus propelling the 

 animal backward. The forward bending of the setse by making it more difficult 

 for them to be forced back beyond the plane of the stiff parts of the fan would 

 seem to make them a more efficient addition to the striking surface. 



The period of life in which the effective tail fan is formed by the libera- 

 tion of the sixth pleopods has hitherto remained unknown for the genus As- 

 tacus. Huxley ('80) says: "I imagine * * * that the appendages of the 

 sixth abdominal somite are at that time (during the first ecdysis) expanded, 

 but nothing is definitely known at present of these changes." Faxon ('8.5) 

 records that specimen of Astacus pallipes 11 mm. long and ten days old still 

 had the sixth pleopods enclosed within the telson and supposed that they would 

 be set free after the second or third moult. Other observations seem to be 

 lacking. 



The telson seen foreshortened in the natural position figure 40, is seen in its 

 true proportions in figure 43. Prom the circular form of the first stage (fig. 20) 

 it has passed through the transversely elongated form of the second stage (fig. 

 23) to its present complex and angular form. By an imperfect transverse hinge 

 it is now divided into an angular anterior part and a rounded posterior part. 

 The fact that this transverse division of the telson does not show till the third 

 larval stage is of interest in connection with the fact that this seems one of the 

 recent acquisitions of the highest crayfish. In the lobsters and other marine 

 forms, as well as in all the crayfish of the Southern Hemisphere, the telson is 

 not at all divided and amongst the Potamobine or higher crayfish of the North- 

 ern Hemisphere the division of the telson is much more perfect in the highest 

 forms, such as Camharus affinis, less pronounced in some lower forms as Cam- 

 barus Clarkii and in Astacus leniusculus, which is doubtless less specialized than 

 Cambarus, the division of the telson in the adult is by no means a perfect one. 



In figure 43 are shown groups of acicular set* symmetrically placed right 

 and left; the rounded, terminal lobe is the only part bearing plumose setae. 

 These plumes stand in a single row and on the right and the left begin ante- 

 riorly as short seta? followed by others that very soon are much longer and of 

 about constant length along the posterior border till near the median line when 

 there is a sudden falling off in length and one very short set;e ends the series. 



