No. 628] ONCHIDIUM AND ADAPTIVE COLORATION 423 



As discharged, the secretion is found to contain at 

 least three elements visible under the microst-opc : a clear 

 fluid, apparently water; clear droplets of oily a])[)('ar- 

 ance; and a finely granular material. The rclativo pro- 

 portions of these three elements vary in different glands. 

 An individual tested in the field, without previous dis- 

 turbance, usually secretes a densely granular mass con- 

 taining a fair proportion of globules ; in the activation, a 

 good deal of the secretion usually adheres to, or falls 

 back upon, the mantle of the snail itself— the dryness of 

 the mantle, and the fact that in nature violent discharges 

 of secretion seem seldom to be invoked are important for 

 the freedom of the mantle from being itself sprayed. On 

 the other hand, the fluids ejected from one or more 

 glands of starved Oncliidia ai-o almost always clear and 

 watery, sometimes with no tract- of the granular or 

 droplet constituents. \'aria1ion in the composition of 

 the corresponding glands of Oncldirlla was recognized 

 by V. Wissel (1898), who also discovered the secretion 

 droplets (globules) which previous workei s liad .)\ci-- 

 looked. 



When the secretion adlieres to the mantle of an Oi/chi- 

 fliniH, it can be seen that considerable stimulation of the 

 skin is thus produced. The secretion is quite acid to 

 litmus and to a number of other indicators. The acid is 

 found in the dense granular material, and does not occur, 

 save in relatively slight amount, when this material is 

 absent. Th.' whit,- ma>ses may he cau-ht on a clean slide 

 and studied under the microscope. The granular mate- 

 rial is insoluble in 90 per cent, alcoliol. or in water. If 

 indicators are added to a dro|) of the si'cretioii under a 

 coverslip, it can be seen that the acid slowly- diffu>es out 



