NO. 10 CRUSTACEAN METAMORPHOSES — SNODGRASS 45 



a change from normal. Inasmuch as Sacculina produces only one re- 

 productive body, the parasite has no concern with what happens to the 

 host. 



Peltogaster socialis, another rhizocephalan, differs from species of 

 Sacculina in that a number of parasites, 2 to 30 of them, all in about 

 the same stage of development, are found on the outside of one host. 

 In his investigation of this species, G. Smith (1906) reported that 

 each external parasite appeared to have its individual root system in 

 the crab. Potts (1915) questioned the accuracy of Smith's observa- 

 tion, and suggested that more probably the several external parasites 

 arise from a common root system, pointing out that Peltogaster 

 socialis is a comparatively rare species and that it would seem unlikely 

 that so many cypris larvae should attack the same crab at the same 

 time. 



That many external reproductive sacs may arise from one internal 

 system of roots has been amply demonstrated by Potts (1915) in 

 his study of the genus Thompsonia. Species of this genus, parasitic 

 on various crabs, reach the ultimate in the conversion of an adult 

 crustacean to the status of a fungus. The parasite within the host has 

 the form of an extensive and intricate network of fine branching and 

 anastomosing threads distributed principally on the ventral wall of the 

 abdomen at both sides of the nerve cord, but also entering the thorax 

 where the branches may extend up on the lateral and dorsal walls. 

 The root threads, according to Potts, are from 10 to 20 microns in 

 thickness. From the central network branches penetrate into the 

 thoracic and abdominal appendages and into the lobes of the tail fan. 



On the branches in the appendages are developed small budlike 

 processes (fig. 17 E, a) that project outward against the integument. 

 These buds contain the germ cells that will become ova. At the next 

 moult of the crab they break through the soft new cuticle and become 

 small external sacs (E, b, and F) standing on the surface. The sacs 

 may be so numerous that the appendages, especially the legs, are 

 loaded with them (fig. 15 B). These external sacs are the reproductive 

 organs of the parasite, and might be likened to the spore-bearing 

 bodies of a fungus nycellium. Since Thompsonia produces no male 

 elements, the eggs are apparently parthenogenetic. They hatch di- 

 rectly into young cypris larvae (fig, 17 E), which, before the next 

 moult of the crab, escape from the sac through an apical perforation 

 (op). The empty sacs are carried off on the exuviae at the following 

 moult of the crab. The development of the eggs, therefore, is so regu- 

 lated that the larvae reach maturity during the time between moults 

 of the host. At each moult a new crop of egg sacs breaks out on the 



