642 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



The swimming legs are the same as in foUaceus and catostomi.^ the 

 first pair only being developed, while the rest are immovable stumps. 

 This first pair (fig. 21) consists of two basal joints well roughened and 

 armed with numerous spines upon their ventral surface, a two-jointed 

 endopod tipped with two short spines, and a one-jointed exopod tipped 

 with two long rowing sette. 



The only difference here from other larvae already described is that 

 the endopod has only two instead of three joints. 



The leg stumps attached to the other thoracic segments all show 



the endopods and exopods 

 clearly, the former being 

 tipped with a single spine, the 

 latter with two. 



In their internal structure 

 the most noticeable differ- 

 ence from other larvte is the 

 almost complete absence of 

 skin glands. 



Fig. 20.— Second maxilliped of the xewly hatched 

 larva of argvlrs americaxus. 



The megalops larva, with 

 its sixty days of inculcation, came forth with a wonderfully well devel- 

 oped system of skin glands; the foUaceus and catostomi larva?, with 

 about half as long an incubation, showed a little more than lialf as 

 many glands. 



And now these americanus larvte, with an inculmtion of onlv seven- 

 teen days, show almost no glands at all. We can not escape the 

 conviction that these glands are developed quite slowly and that they 

 do not appear until comparatively late in larval life. It would seem 

 also that they must be de- 

 veloped independently of 

 the incubation period, so 

 that while the latter is 

 changed greatly, being 

 doubled in some species and 

 halved in others, the glands 

 apparently alwa3^s require 

 about the same time for de- 

 velopment. In these americanus larva? we find but a single small group 

 of glands, six or seven in number, on either side near the posterior 

 edge of the carapace lobes. They are much smaller than in megalops^ 

 and the ducts are not at all distinct. There are also a few scattered 

 glands along the dorsal surface of the carapace, thorax, and abdomen, 

 but they are all very small, and none of them show the structure given 

 for foliaceus and megalops. They have rather the appearance of being 

 in an immature and partiall}" developed condition. 



The paired shell glands are invisible, due to the opacity of the 



Fig. 21. — First swimminc* leg of the newly hatched 



LARVA OF ARGULUS AMERICANUS. 



