THE CIRRIPEDIA 



131 



visceral mass by a narrow mesentery (//tw), near to wliich on eacli 

 side are the paired (more rarely unpaii'ed) openings of the male 

 and female generative organs. In the ditlerent genera the external 

 form varies considerably, and with it the position of the mesentery 

 and of the genital apertures. In Peltogaster (Fig. 81, B), which 

 may be regarded as the most primitive form, the body has an 

 elongated sausage-shape, with the mantle-opening at one end, and 

 is attached by the peduncle about the middle of its length. The 

 mesentery is longitudinal on the proximal side (next the peduncle). 

 The genital apertures are placed on each side close to the mesentery, 

 the female openings being nearer the end where the opening of the 

 mantle is situated. Comparison with a normal Cirripede (Fig. 81, 

 A), especially as regards the position of the genital apertures, 

 suggests that the mesentery is on the dorsal side, and the mantle- 



Fin. 81. 



Dia-iam to illustrate comparison of RhizocRphala with norinal Cirripede. A, Lepas; B, 

 Pcltogustir ; C, Sun-nUnn. »/i, mantle ; j/k.-;, mesentery ; ?i, nerve-^'aiiglion ; o, opening of mautle- 

 i'avity. (J, male ^'fiiorativc aiierture ; 9, female ,i;eiierative aperture. 



opening at the anterior (or rostral) end. In Scuxulim (Fig. 81, C), 

 which is parasitic on Brachyura, the whole body is flattened 

 in the plane of the mesentery, and has assumed a secondary and 

 superficial l)ilateral symmetry about a plane at right angles to 

 this and coinciding with the median plane of the host. The 

 mantle-aperture is in the middle of the distal edge, and the 

 mesentery has suffered a corresponding displacement, extending 

 from the mantle-opening to the point of attachment of the peduncle 

 on the side which, in the natural position, is turned towards the 

 right side of the host. The genital openings, except that they are 

 more Avidely separated from each other, occupy the same relative 

 positions as in Pdlorjmter. In other genera, such as Lemaeodiscus 

 and Triangulus, the symmetry becomes still more complicated, and 

 in Clistosacciia and Si/lou the genital organs are unpaired. 



The peduncle perforates the integument of the host and gives 



