NO. 10 CRUSTACEAN METAMORPHOSES — SNODGRASS 49 



last three segments (B) ; the segmental limits disappear owing to the 

 unfolding of the previously deeply infolded intersegmental mem- 

 branes. This is the feeding stage of the parasite (B, C), called the 

 pranize by Monod (L., prandium, lunch). Its meal lasts about six 

 months. 



At the end of winter or the beginning of spring the fully fed para- 

 sites leave the host and return to the bank of the estuary. The males 

 individually dig burrows or take possession of empty ones in advance 

 of the coming of the females. The completed burrows are 1.5 to 

 2.5 cm. in depth, sloping downward from the mouth to an inner 

 chamber 4 or 5 mm. in diameter. When the females arrive they enter 

 burrows already inhabited by a male; as many as 10 or more may 

 consort with a single male. Within the burrows both the male and 

 the females undergo their first and only moult, accompanied by a 

 small degree of metamorphosis. The cuticle splits crosswise over the 

 thorax, and the two ends are cast off separately. The sexes are now 

 differentiated and the isopods enter their third functional stage, which 

 is that of reproduction. The male (fig. 18 D) retains a relatively 

 slender figure, but the female (E) becomes greatly distended with 

 the development of the ovaries. The mouth parts of both sexes are 

 reduced, except the mandibles of the male (F), which are long prongs 

 perhaps used for digging or for holding the female in mating. Sub- 

 sistence is now at the expense of the food consumed during the para- 

 sitic stage. 



The eggs develop into mature larvae within the ovaries of the fe- 

 male, which become distended into a pair of large, saclike uteri, com- 

 pressing the empty alimentary canal between them. On the ventral 

 surface of the female's thorax are several pairs of small overlapping 

 oostegite plates, and above them is a large atrial cavity, into which the 

 oviducts open, but this cavity does not serve as a brood chamber. 

 When the young issue from the uteri through the oviducts into the 

 atrium, the oostegites open and the larvae precipitate themselves head 

 first through the aperture directly into the water, where they at once 

 begin active swimming. After giving birth to the young, the females 

 quickly die, but the males are longer lived and their metamorphosis 

 is not so closely correlated with the season. 



There is clearly in the life history of Paragnathia formica little that 

 can be called a true metamorphosis. The change of form between the 

 two larval phases is merely a distention and elongation of the thorax 

 resulting from the unfolding of the intersegmental membranes. The 

 metamorphosis at the moult to the adult stage involves principally a 

 reduction of the mouth parts which are no longer used for feeding. 



