STUDIES OX THE EC'TOrAUASITK' TI^EM ATODES OF JArAX. jOl 



surface views. The niiinerons dncts tli:it ])r()coe(l from endi Imlf 

 of t1ie vitellariiiin at last unite witli one another and foian a sinîi'lo 

 larii'e duct, tlie paired yolk-dnct. Thesj are in most species sym- 

 metrically dis])osed with respect to the uiediun line of tlic hody, 

 l)ut in Hexacotijh' thai of tlie ri^'ht side is sitnated nuich more 

 in front of the left (PI. XrA\ fig's. 1 .^ 7). Tn 1 )irli,h,i,]i<>ni a 

 similar asynuneti'v is also ohservahle, hut is not so strikinu' (IM. X, 

 fig's. 1, 5, & !)); and in this genus the relati\e ])ositions of the ducts of 

 the two sides of the Ijody are the reverse of what is found in 

 HcxacoiijJt', the duct of the left side heing situated anterior to 

 its fellow of the o])])ositi' side. During the sumuier season the 

 ])aire(l volk-ducts are almost always filled Avith yolk-cells, and 

 are eonsequentiv veiw eonspiouous. heing sometimes pei'ce])til)le 

 Avith tlie naked eye through the transparent tissue of the W(^rm. 

 In Axiue hetcvoccrca, ]\ficrocof)ih\ He.vacotiih', and OncliocotijJc they 

 run for the greater ])art of their lengths p:u'allel to the hnig axis of 

 the l)odv, just on the inner side of the intestinal trunks; hut in Cidi- 

 (''ili/le, j\fonocr)tiih\ TrixtOïiiinii, EpihiJcIhi, and ] )icJiiJo])]ior<i the ducts of 

 hoth sides are situ:!ted on the same sti'aight line, which is dii'ccted at 

 a right angle to the long axis of the hodv. Tn Axiiii' iilx-rniiis and in 

 ( Ifiiicotijle the paire(l ducts unite with each othei' in the nu'(li:!n line of 

 the hody immediately after leaving the vitellarium, and ii)rm the 

 single 3^olk-duct (PI. VTI, 1ig. 5). In Tn'stomiiiii, CdHcoiijh', and Mono- 

 cotifle the paired volk-duct of each side is again formed hy the union 

 of two smaller ducts Avhich come respectively from the anterior and 

 posterior part of the A'itellarium; sometimes, however, it is formed 

 hy the simultanec^us union of numerous ducts, as in 7V/.sY. ovale 

 (PI. XXIII, fig. 1). In M/crncofij]/' tniucnta the pairetl yolk-ducts a.ppear 

 to unite with each other at the very beginning and then to se[)arate again 

 into two ducts similar to those of the other species (PI. It. fig. 1); 



