NO. 1788. NORTH AMERICAN ERGASILID^— WILSON. 321 



ganglion, which Hes above it and which is heavily pigmented with 

 black. 



The balancers at the posterior end of the body are cyhndrical, 

 very slender, and slightly curved, but there is no trace of the spatulate 

 form so characteristic of the Caligidae. They are attached on the 

 dorsal surface some little distance in front of the posterior end of 

 the body, and project obliquely backward and outward. Between 

 these balancers and projecting from the posterior end of the body 

 is a large hemispherical knob, slightly flattened dorsoventrally. 

 This, combined with the symmetrical elliptical outline, gives a body 

 form quite unlike that of the Cahgidae, but very similar to that of 

 Cyclops nauplii. 



The body itself is simple and without segmentation, and consists 

 of a cellular exterior surrounding the general body cavity, through 

 the center of which passes the digestive tube. 



In the Caligidse the anterior portion of the body of the naupUus 

 is transparent, while the posterior half is rendered opaque by the 

 presence of numerous yolk cells. There is hardly more than a trace 

 of a digestive tube, and the pigment is distributed along the lateral 

 and posterior margins of the body in various patterns. 



Here in the Ergasilidse, on the other hand, the whole body of the 

 naupHus is equally transparent, and all the pigment except that 

 appearing in the supraesophageal ganglion is confined to the walls 

 of the digestive tube. The cellular exterior is not quite as clear as 

 in the Cahgidae, but the individual cells can be more plainly seen. 



The fluid which represents blood and wliich circulates in the body 

 cavity is also more distinctly visible. Along the sides of the body 

 can be seen the muscles which move the appendages, each extending 

 backward obliquely from the base of the appendage nearly to the 

 posterior end of the body; they are all plainly striated. 



The digestive tube is much more distinctly visible than in the 

 Caligus nauphus and fills the center of the posterior half of the body. 

 To make it still more prominent all the pigment, as just stated, is 

 confhied to its walls, which thus stand out prominently in contrast 

 to the colorless tissues surrounding them. It is enlarged at the 

 anterior and posterior ends into a more or less spherical sac, and 

 constricted in the intervening space. 



Not only is it thus more visible, but it is better developed and the 

 peristaltic movements of its walls are plainly in evidence. No 

 opening could be seen at its posterior end, and when, in consequence 

 of the increasing pressure of the cover glass, the body finally burst 

 and its contents escaped the rupture took place at one side of the 

 intestine and not at its end; and the intestine itself was not rup- 

 tured nor did its contents pass out for a long time. This is good 



Proc. N. M. vol. 39— 10 23 



