272 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. 39. 



In the present family the fifth segment is considerably the smaller and 

 is usually well separated from the sixth. But in a few species the two 

 are thoroughly fused, yet even here the fusion is still indicated by the 

 presence of the fifth legs attached to either side of the compound seg- 

 ment near its anterior margin. 



The sixth segment is less worthy of bearing the name "genital" 

 than it was in the Caligidse. 



In that family it contains the convolutions of the oviduct within 

 which most of the development of the eggs takes place, the large 

 cement glands which furnish the material for the external egg cases, 

 and the sperm receptacles from which the eggs are fertilized as they 

 pass out into the cases. 



Here in the Ergasilidse it apparently contains nothing but the 

 posterior portion of the sperm receptacles and a somewhat compli- 

 cated musculature which controls the 

 openings of the oviducts. 



The convolutions of the oviducts and 

 the cement glands are situated farther 

 forward in the free thorax and cepha- 

 lon. There the eggs are matured, and 

 v/hen ripe they simply pass out through 

 the genital segment one by one, with- 

 out remaining in it for any length of 

 time. And yet, understanding this, 

 it is better to retain the old name 

 and thus avoid confusion. In this 

 family, therefore, the genital segment 

 approaches more nearly to the struc- 

 ture of the other thorax segments and 



Fig. 3.— Dorsal surface of the genital 



SEGMENT OF ErGASILUS CENTRARCHIDA- 

 RUM, showing THE COMPLEX MUSCULA- 

 TURE, a AND C, CLOSING MUSCLES; b AND 

 d, OPENING MUSCLES. 



varies but little more than they in 

 shape and size in the two sexes and in different species. 



It is somewhat enlarged, to be sure, but never as much as in the 

 Caligidse, and is fairly uniform in shape throughout the group. It is 

 relatively smaller in the males and immature females, but the differ- 

 ence in size is only trifling. 



Accordingly, there is not much danger that the careful systematist 

 will mistake an immature female for a male, as has frequently been 

 done when dealing with the Caligidae. 



And there is not as rigid a demand for absolute precision as to the 

 stage of development or the degree of maturation when comparing 

 different specimens for purposes of classification. The eggs are 

 arranged in several longitudinal rows, the number varying in different 

 species. The length of the rows and the shape of the sacks also varies, 

 but in general the sacks are club-shaped, larger at the posterior end, 

 and are very similar to those of Cyclops. 



