380 



GILMAN A. DREW. 



force some of tbe blood out of the blood-spaces of the sus- 

 pensory membranes. The movements are not enough to form 

 strong currents of water, such as are formed by Yoldia (1) . 



The margins of the lobes of the mantles are never pro- 

 truded far beyond the margins of the valves of the shell, and 

 the pallial muscles are accordingly not excessively developed. 



Each of the large palp appendages is supplied with a 

 rather large muscle that is continued into it from the body- 

 wall. It occupies the ventral (morphologically outer) side of 

 the appendage (fig. 66, Im.), and is continued to its tip. This 

 muscle serves to retract the appendage. Its position in the 

 appendage is such that when the appendage is strongly 

 retracted it is curled as shown in fig. 48. The muscle seems 

 to be homologous with fibres that extend into the membrane 

 that suspends the palps from the body-wall. 



Excretory Organs. 



Just before embryos reach the stage where the second gill 

 filaments begin to flatten, preparatory to forming the third 

 gill filaments, a pair of narrow tubes appear just anterior to 

 the visceral ganglia and ventral to the pericardium. The 

 two tubes touch each other on the median line of the body, 

 but their cavities do not seem to communicate. Laterally 

 they are extended to the surface of the body, where they open 

 into the mantle chamber. This is the earliest stage in which 

 I have been able to distinguish the kidneys. I have not 

 succeeded in determining whether the external openings are 

 present from the beginning, or whether they are formed later. 

 I am inclined toward the view that the kidneys are meso- 

 dermal in their origin ; but this view is based simply on the 

 length and narrowness of the tubes when they can first be 

 distinguished. They may be formed as invaginations from 

 the surface. 



The cells forming the walls of the kidneys soon become 

 large and vacuolated. This character is retained throughout 

 the life of the animal, and makes the tracing of their cavities 



