16 JOHNS HOPKINS UNIVERSITY MOEPHOLOGICAL MONOGRAPHS. 



are much longer than during contraction, and lie in curves, Fig. 3, 

 g. When the suspensory membranes contract, the longitudinal muscles, 

 Fig. 19, ulm., and llm., also contract, the gills are shortened, and the 

 plates are more closely opposed. 



Each plate is rendered rather rigid ventrally by chitinous rods 

 that are so disposed as to allow flexibility. Many of the muscle 

 fibres of the plate are attached to these rods in such a way as to 

 insure an even strain, and keep the plate from bending ventrally 

 when pressure is applied. 



Experiments were tried to determine, if possible, the part taken 

 by the gills in the collection of food. For this purpose both mutilated 

 and uninjured specimens were used. No definite results were reached, 

 but they were not observed actively engaged in collecting food. Con- 

 sidering the remarkable activity of the palps as collectors of food, 

 such activity for the gills seems rather unnecessary, and it would 

 also seem that the pumping action of the gills would seriously 

 interfere with their normally performing such a function. 



As an experiment, specimens were placed in shallow dishes and 

 supplied with water in which living organisms were very abundant. 

 Under these conditions many specimens died with all the symptoms 

 of starvation, and those that still survived after several weeks of this 

 treatment were very weak and without the usual color. These weak 

 specimens, when placed in their native mud, where the palp-append- 

 ages could be used, regained their strength and color very rapidly. 



EXCRETORY ORGANS. 

 FIGURE 23. 



The excretory organs of the adult animal are very extensive. They 

 consist of many loops, the walls of which are much sacculated. They lie 

 ventral to the pericardium, and are continued into the foot along the 

 posterior side of the stomach. The inner, pericardial, opening of each 

 excretory organ is separated from the outer, mantle-chamber, opening 

 by a very short space. 



Fig. 23 is a drawing of a wax model of the ends of the left excretory 

 organ and genital duct, seen from the ventral side. The inner end, ie., 

 opens into the pericardium, pc., by a rather large opening, turns 

 abruptly towards the middle line of the body, crosses the outer end, oe., 

 and is continued anteriorly close to the pericardium. After passing 



