PARASITISM, ETC., ON THE SEA-SHORE 143 



portion of the parasite which protrudes consists mainly of 

 the reproductive organs surrounded by a " mantle," which 

 has a small opening surrounded by a sphincter muscle (see 

 Plate XIV). The visceral mass is fixed to the host by a 

 short stalk which penetrates the abdomen and gives off a 

 number of branching roots that serve to absorb nourish- 

 ment from the crab's tissues. 



With regard to the development, the larva escapes from 

 the mantle-opening as a nauplius. Subsequently it passes 

 through a cypris stage, and then eventually attaches itself by 

 one of the antennules to the base of a seta situated on any 

 readily accessible portion of the crab's body. After under- 

 going a series of profound changes the larva penetrates the 

 soft cuticle at the base of the seta by means of a specially 

 developed boring organ, and gets into the blood stream. 

 By this means it is carried to the under side of the intestine, 

 where it becomes attached. The fact that the crab carries 

 its abdomen closely apposed to the under side of the thorax 

 makes it practically impossible for the larva to enter at or 

 near the point where the adult is seen to emerge, and the 

 larva can only reach this position in the manner described. 

 (See Delage, 1884, and Smith, 1906.) 



Both Sacculina and Peltogaster (a form very similar to 

 Sacculina, which attacks hermit crabs) exert an important 

 influence on their hosts. In particular, the effect of the 

 parasite upon a male crab is to convert it towards the female 

 condition. Externally, the resemblance of a parasitised 

 male crab to a female is practically complete. The copu- 

 latory styles, which are the only abdominal appendages of 

 the male, give way to the normal swimmerets of the female. 

 Internally, the effect is to cause the testes to degenerate and 

 to destroy their functions. Not least remarkable is the fact 

 that, if by some means the parasite should drop off its victim, 

 the cells of the testes still remaining give rise not to sperms, 

 but to ova. Smith (19 13) considers the effects upon the 

 male crab are due to the roots of the parasite affecting the 

 crab's metabolism in the same way as does the developing 

 ovary, viz. by their taking up fat and stimulating the ovary 



