344 



THE INVERTEBRATA 



side of the intestine (Fig. 251). There rootlets begin to grow out 

 from it and eventually permeate the body of the crab to the extremities 

 of the limbs. Meanwhile a knob also grows from the mass; forms 

 within itself a mantle cavity surrounding an internal "visceral mass" 

 which contains the rudiments of genital organs and a ganglion ; presses 

 upon the ventral integument of the abdomen of the host, whose 

 cuticle is thus hindered from forming at that spot ; and consequently 

 at the next moult of the crab comes to project freely under the 

 abdomen, where it may be found in the adult condition. 



The phenomenon known as parasitic castration is exhibited by 



B 



A, NaiipUus. B, Cypris. 

 undifferentiated cells ; 



Fig. 247. Larval stages of Sacculina. From G. Smith. 



A.i, antennule; A. 2, antenna; Ab. abdomen; E. 



F. frontal horn with gland cells; Gl. gland cells; Md. mandible; Ten. frontal 



tentacles (frontal organs); Tn. tendon. 



crabs attacked by SaccuUna. The moult at which the parasite becomes 

 external produces a change in the secondary sexual characters in the 

 new cuticle. The male crabs have a much broader abdomen, reduced 

 copulatory styles (these may disappear altogether), and abdominal 

 swimmerets (which carry the eggs in the female, and are absent in 

 the normal male). There is, in short, a marked tendency to the female 

 type. In the female crabs there is also a change, but this is held to be 

 not towards the male but towards the juvenile type. The gonads dis- 

 appear, but cases have been observed in which the parasite has been 

 killed and months afterwards what was probably an originally male 



