178 Proceedings of the Royal Physical Society. 



They liave, moreover, had to become as strong as possible, in 

 order to resist the rough usage to which they are necessarily 

 exposed, and have therefore become cylindrical, and have 

 greatly thickened their walls at the expense of the lumen. 

 They have also greatly adapted themselves to the circumstances 

 of their position in regard to length, stoutness, and amount of 

 aggregation ; the details of this adaptation have been given 

 elsewhere. I need scarcely say that the intermediate forms 

 above described are not to be found in the adult Astacus ; it 

 would be interesting to seek them in young stages. I append 

 a tree presenting in tabular form the result of the reflections 

 given above : 



Auditory Seta, 



Olfactory Seta. 



Frincring Seta. 



I 

 Primary Tactile Seta. 



I 



I I 

 Primitive Seta. 



With regard to this loss of secondary bristles in the tactile 

 setae, it is interesting to compare the figures of Grobben, 

 Studer, Weismann, and Glaus. Grobben {loc. cit, p. 441) 

 describes the tactile setae of the larva of Ptychoptera (see 

 PL VIII., Fig. 10) as " very long, and generally split into 

 from two to five parts," and then describes the nervous 

 apparatus, which is the ordinary primitive one of a nerve- 

 fibre with a single ganglion-cell. 



Weismann (loc. cit.) says that the tactile setae in the larva 

 of Corethra are " simple or feathered," and one can pick out 

 from the setae figured (Taf III., Figs. 3-7) a series (which I 

 reproduce in PL IX., Fig. 11, a-e) which leads from a seta 

 feathered on one side (a), which resembles the olfactory seta 

 of Thysanoijoda (without the flattening), or half of one of the 

 setae figured by Leydig in Corethra (possibly Weismann's 

 figures are from a later stage, when, after a moult, perhaps, 

 the setae were beginning to lose their primitive character), to 

 one like Grobben's figure. Studer, again, has given in some 



