16 



General Morphology. 



we may say that in Lernaeodiscus this surface is greatly elongated and laterally extended, and 

 shows an immense curvature, owing to the antero-posterior compression of the body. Since the 

 mesentery and mantle opening are both situated in the middle line relatively to the crab, 

 the body has the appearance of bilateral symmetry; but this is deceptive, as the peculiar 

 positions of the genital openings show. These are situated in an asymmetrical position, the vas 

 deferens and oviduct of one side opening on the anterior hinge of the mesentery, those of 

 the other side on the posterior hinge (Plate 7 tigs. 33 and 34). The condition of Lernaeodiscus 

 can very simply be derived from that of Peltogaster by first slipping the latter, without altering 

 its symmetry relatively to the host, onto the median ventral line. We than take the Pelto- 

 gaster and without twisting its axis bend the anterior portion of the body lying in front of 

 the ring of attachment outwards towards the posterior pole. In this manner the mesentery 

 will become bent into an hinge as in Lernaeodiscus, and the mantle opening will face towards 

 the posterior pole of the host instead of anteriorly as in Peltogaster. Now with regard to the 



crp 



uvd I 



ovd r 



vrd r- 



,rcmcL 



Text fig. 7. 



asymmetry of the genital openings: it has been already noted that in Peltogaster the openings 

 on one side are always slightly in advance of the other side. We may illustrate this diagram- 

 matically by Text fig. 7 A which represents the mesentery of Peltogaster viewed ventrally 

 with the genital openings upon it, those of the morphologically left side being in advance 

 of those on the right. 



If we exaggerate this condition as shown in Text fig. 7 B, the bending of the mesentery 

 into an hinge at the ring of attachment will have the result of carrying the openings of the 

 morphologically left side onto the anterior hinge of the mesentery [A), and those of the right 

 side onto the posterior hinge (P), as shown in Text tig. 7 C, and this is exactly the condition 

 of Lernaeodiscus. 



The allied genus Triangulus differs from Lernaeodiscus in much the same way as Hetero- 

 saccus differs from Sacculina, namely in the suppression of part of the mesentery. In Trian- 

 gulus the anterior arm of the hinged mesentery, namely that arm which should connect the 



