ANATOMY OF STYLETS OF CAMBARUS AND ASTACUS. 89 



From the state found in this Astacus, the more "extinguisher-" 

 like shape found in the English Astacus could be formed by a 

 process of simplification, or reversal of specialization, just as is 

 true for the first stylets. The above mentioned larvae of Astacus 

 leniuscuhis furnished but meager facts bearing upon the ontogeny 

 of the second stylet, but this is enough to establish the existence 

 of an early modification of the median edge of the endopodite to 

 subsequently form the triangle or scroll. The second pleopod 

 is at first like the following ones and only gradually takes on the 

 specializations that make it an accessory sperm-transfer organ. 

 The earliest detected modification of the endopodite was a slight 

 groove followed in larger larvse by an elevation on which the same 

 groove was seen. How this groove on an elevation gives rise to 

 the triangle remains for study of later stages to decide. 



In Fig. 27, which is the anterior face of part of the second 

 pleopod of a male, there is a marked protuberance on the median 

 side of the endopodite, and this contains a lateral groove. In 

 some other males the groove was present, but not the elevation. 

 Thus Figs. 25, 26 represent the anterior and the posterior faces 

 of the edge of the endopodite of a male 23 mm. long, showing 

 only the exoskeleton and the plumose setae. The groove is a 

 transverse pit which ends abruptly on the anterior face of the 

 endopodite ; it is bounded distally by a slight lip-like transverse 

 ridge standing out into the water. 



There is thus a transverse pit on the median face of the endo- 

 podite at the region that will later be part of the triangle (com- 

 pare Figs. 27, 24). The cells of the epidermis were small and 

 ran in as a single layer to line the pit and extend into the lip as 

 a solid mass. 



In the more advanced stage Figs. 27, 28 this same pit is on a 

 decided elevation. The pit is a transverse slit still lined with 

 epidermal cells (Fig. 28), but its distal edge is no longer a lip but 

 only part of the general elevation. This male was the one having 

 the advanced first stylets seen in Fig. 23. The posterior view 

 (Fig. 28) is intended to show the epidermal cells in surface view as 

 well as in optical section and also the fact that the pit opens 

 gradually onto the general level on this posterior face, while on 

 the anterior face it ends abruptly at a steep wall lengthwise of 

 the endopodite. 



