THE GENUS LEEN^ODISCUS. 431 



the arrangement in L. galathece. Corrected diagrams of the anatomy of the 

 latter are given in figs. "5 and 6. It will be seen from these diagrams that 

 the positions of the testis-openings are the same as in L. munidce, and not as 

 T originally figured them in my monograph (3, PL 7. fig. 33). The nerve- 

 ganglion is similarly situated on the large right mesentery, though a little 

 further away from the mantle-opening than in L. munidce. The two oviducal 

 openings, one on each mesentery, are decidedly further away from the 

 mantle-opening than in L. munidce, and are also more definitely on the 

 mesenterial edge and not on the wall of the visceral mass as in L. munidce. 

 How much importance is to be attached to these differences cannot he 

 definitely stated, as the greater or less distension of the visceral mass mio-ht 

 account to some extent for the shifting in position. 



Another point of difference between L. munidce and L. galathece is that in 

 the former all the specimens hitherto obtained have the mantle-openino- very 

 definitely situated in an asymmetrical position on the right anterior corner of 

 the body, while in L. galathece this opening is sometimes situated medianly 

 or elsedeflected to the right anterior corner to a less extent than in L. munidce. 

 The fact that in L, galathece the position is variable and sometimes practically 

 the same as in L. munidce, should make one hesitate before afctachino" much 

 importance to this slight point of difference. 



In explaining the peculiar orientation of this parasite it is necessary to 

 bear in mind the orientation of a more normal member of the Rhizocephala, 

 and for this purpose Peltogaster may be chosen because its relations of 

 synnnetry are very simple. Fig. 7 is a diagram of the mesentery of Pelto- 

 gaster viewed from the peduncular surface, or surface of attachment. This 

 surface, as I have shown elsewhere (2), is the dorsal surface. The median 

 axis of the body passes through the line AB, A being anterior and B posterior. 

 The mantle-opening {op.') is seen at the anterior end of the mesentery, and 

 the peduncle (p.) is seen piercing the mesentery somewhat toward its 

 posterior end. The nerve-ganglion {n.) is seen on the anterior portion of 

 the mesentery, and the genital openings are distributed on each side of the 

 mesentery, the oviducal openings being in front and the testicular openings 

 behind. The openings of the left side are a little in advance of those on the 

 right side. 



The way in which Lernceodiscus can be derived from this symmetrical 

 condition is shown in fig. 8. Here the original long axis passes throuoh AB 

 and the testes still occupy their original position, but the mesentery has been 

 expanded laterally, especially on the right side, and the anterior portion of 

 the mesentery in front of the peduncle has been rotated to the right, so as to 

 be nearly at right angles to the original long axis AB. The position of the 

 nerve-ganglion, of the oviducal openings, and of the mantle-opening relatively 

 to the testes and the peduncle, clearly show that this rotation has tf<.ken 

 place. 



