SPICULES OF SYNAPTA AND AUPvICULAPviA LAEVA. 



49J 



dinally-wriukled conclitiou (text-fig. 3). If, further; we suppose 

 that, in some way, the rods are caused to project on the 

 exterior by one extremity only, the pocket-like folds will be 

 rendered still more conspicuous (text-fig. 4). These condi- 

 tions are those found in the case of the adult spicules oE 

 Synapta, the last possibly being brought about by the con- 

 tinued attachment of the knob end of the primary anchor rod 

 to the dermal epithelium from which it oi-igiuated, and by 

 the connection of the handle extremity of the shaft with the 



Text-i'IGS. 2, 3, 4, and 5. 



m, 



4 5 



These diagrams illustraie the formation of the dermal pocket whicli 

 envelops the anchor-bow described in Die text. In flg. 2 the rod is 

 lying tangentially on the nncontracted circular muscle-layer of the 

 animal ; in fig. 3 the circular muscle-layer has contracted, and in 

 consequence tlie outer dermal layer is thrown into longitudinal folds 

 and forms two pockets at the extremities of the rod ; in fig. 4 the 

 anchor protrudes on the surface at one extremity only, wiih the 

 result that one large dermal pocket is formed ; in fig. 5 the anchor 

 and dermal pocket enveloping it are viewed from the surface, though 

 they are supposed to be foreshortened. 



plate, whicli lies internally and parallel with the muscular 

 layer (see text-fig. 1).^ The knobbed extremity of the anchor 



• I cannot guarantee that the position of the spicules relative to the 

 dermal- and muscle-layers represented in figs. 11, 12, and 15 is exactly that 

 which obtains in nature, since the process of section-cutting is liable to shift 

 the spicules. 



