14 



General Morphology. 



shifting its position onto the ventro-median line of a symmetrical crab, the mantle opening 

 would be situated in the most disadvantageous position, namely at the immediate junction of 

 thorax and abdomen, where the flexure of the host's body would not admit of free passage 

 through the mantle opening. Now this is exactly what has been avoided in all Rhizocephala 

 by an alteration of the symmetry of the body relatively to the crab, and in some cases by the 

 assumption of a definite asymmetry in the body itself. 



It appears that this has been effected in two ways, one method producing the genera 

 Parthenopea, Sacculina and Heterosaccus, the other method producing the curious asymmetry of 

 Lernaeodiscus and Triangulus, 



In the first of these two groups the body has been simply as it were rotated on its 

 peduncle or ring of attachment so as to bring the mantle opening from the anterior position 

 of Peltogaster to a lateral or posterior position, and concomitantly the ring of attachment has 

 shifted from the middle of the mesentery to its posterior extremity. The first stage in the 

 process is represented by Parthenopea (Plate 1 fig. 7) a parasite found on the ventral surface 

 of the thorax of the Macrurous genus Callianassa. The appearance and anatomy of this form 

 strongly recalls Peltogaster; as in Peltogaster the root system is green and the external part 

 of the body is red, but the roots are more diffusely branched and the oviduct is slightly com- 

 plicated into a branched colleteric gland. 



ovd.T- 



xrd.-f- 



'™f 



o8.{ 



t-unji. 



CvdL.L 



The external part of the body is more or less spherical, and the mantle opening is 

 situated laterally relatively to the host, being either on the right or left side. The long axis 

 of the mesentery, which stretches from the ring of attachment to the mantle opening is ver- 

 tical to the long axis of the host, so that the length of the mesentery is only seen in a 

 lateral view of the host. The genital openings are situated symmetrically about the mesentery 

 Text fig. 5B. 



Parthenopea, intermediate in its anatomical characters between Peltogaster and Sacculina, 

 and with its axis of symmetry shifted onto a vertical and lateral position on the ventral surface 

 of its symmetrical host, gives the clue to the condition found in Sacculina. 



