472 G. H. PARKER 



of the siphonoglyph in Metridium is firm, almost as stiff as a 

 thin layer of cartilage, it is quite clear that these muscles are 

 so concentrated as to exert a tension favorable to folding the 

 siphonoglyph walls when the animal contracts. The parietal 

 muscles extend up and down the length of both sides of the 

 mesenteries next the column wall. This, like the wall of the 

 siphonoglyph, is somewhat resistant to folding and the parietals 

 are undoubtedly especially concerned with drawing it together 

 in general contraction. Thus the longitudinal muscles and the 

 parietals show a certain amount of specialization adapted to 

 their particular tasks and yet constitute a single physiological 

 system for the depression of the oral disc. 



The means by which these muscles can be brought into action 

 are extremely diverse. If a Metridium is kept in running seawater 

 in the dark, it soon attains to its fullest expansion. Under such cir- 

 cumstances its height may be six times the diameter of its column. 

 If now it is suddenly illuminated by diffuse daylight or a strong 

 electric light, it will gradually shorten its column to about one 

 third or one fourth its former length, but without contracting 

 its oral disc. This operation is accomplished by the simul- 

 taneous and united action of the longitudinal and parietal mus- 

 cles and, so far as can be seen, necessitates the cooperation of 

 no other muscles. In this change of form some of the water 

 in the gastro vascular cavity is discharged through the niouth; 

 in consequence of the retention of the rest the diameter of the 

 animal is increased, thereby putting the circular muscle of the 

 column under a certain tension. Recovery from this state is 

 more gradual than its assumption and is dependent partly on 

 ciliary action whereby water is returned from the outside through 

 the siphonoglyphs to the gastrovascular cavity, and partly by 

 the action of the circular muscle of the column, which by pressing 

 on the fluid contents of the gastrovascular cavity thus pushes 

 the oral disc away from the pedal disc. The return to the ex- 

 panded state is doubtless also in part dependent upon the re- 

 laxation of the longitudinal and parietal muscles, a condition 

 which comes about when the animal is in the dark in running 

 seawater. Whether there is any reciprocal relation of a more 



