NO. lO CRUSTACEAN METAMORPHOSES — SNODGRASS 43 



rootlike processes {rhizai), which continue to grow, branch, and unite 

 until a network surrounds the intestine (B), from which branches 

 penetrate between the other organs and extend out into the append- 

 ages. The roots do not enter the tissue of the crab, but Delage says 

 only the heart and the gills are not attacked. These are the organs 

 necessary for maintaining the life of the host and therefore that of 

 the parasite, but how did the parasite ever learn to discriminate ? The 

 Sacculina at this stage has been aptly likened to a fungus. That a 

 crustacean can be so transformed shows the unlimited potentialities 

 of metamorphosis. 



When nutrition has been fully provided for, attention must be given 

 to the reproductive function. If the eggs were allowed to hatch inside 

 the crab, the young larvae would find themselves in a prison from 

 which there would be no escape. The body of the parasite, therefore, 

 emerges through the ventral integument of the crab and becomes a 

 brood chamber in which the eggs mature and from which the larvae 

 are liberated into the ocean. The pressure of the parasite's body 

 causes a dissolution of the crab's epidermis beneath it, and prevents 

 the formation of cuticle at this point. Consequently at the next moult 

 of the crab the Sacculina body containing the reproductive cells 

 emerges and becomes external, but is still connected with the crab 

 by a short peduncle giving passage to the feeding roots. The place of 

 emergence is at the middle of an abdominal segment ; if it were inter- 

 segmental, movements of the abdomen might constrict the peduncle 

 and shut off the food supply of the parasite. It seems that the simpler 

 a creature may be in its organization, the more does nature guard 

 it against emergencies. It is interesting to note that the species shown 

 at A of figure 15 is exactly modeled to fit into the pocket between the 

 under surface of the thorax of the crab and the reflexed abdomen 

 beneath it. 



The external parasite, as seen in section (fig. 17 C, D) consists of 

 a central mass of cells contained in a tunic suspended from the 

 peduncle, and of an outer mantle {mn) that encloses a peripheral 

 brood chamber {he). The figures at C and D, taken from G. Smith 

 (1906), depict a species of Peltogastcr, but the structure is essentially 

 the same in Sacculina. The cells of the central mass are the eggs in 

 the ovary {Ov) ; above them is a pair of tubular testes {Tes) and a 

 single nerve ganglion (Gng). The ripe eggs are discharged into the 

 mantle cavity and here fertilized by spermatozoa from the testes, the 

 parasites being necessarily hermaphroditic. The larvae escape in the 

 cypris stage from an opening (D, op) in one end of the brood 

 chamber. Successive lots of eggs are discharged and fertilized, and 

 after each brood of larvae the cuticular lining of the brood chamber 



