52 



THE ROYAL SOCIETY OF CANADA 



some of the older forms were so far developed as to give some indication 

 of what might be expected in the succeeding stage. 



Stage I. The outline of the carapace, which is the most distinctive 

 feature of this type of larvae, is in general oval, but there is a certain 

 amount of variability observable, the greatest transverse axis being 

 situated in some examples at about the middle of the length, in others 

 some distance in front of it, so much so occasionally as to give a kite- 

 shaped outline. Similarly, while in general the greatest width of 

 the carapace is about half its length, variations occur, some individuals 

 having a more slender form than others. The dimensions of six 

 examples taken at random were as follows: — 



Greatest breadth 

 " " 





 

 

 



24 mm. 



23 



20 



20 



20 



225 



The carapace (Fig. 7) presents the appearance of being composed 

 of a number of plates of varying shapes, but accurately fitted 

 together so as to produce a symmetrical pattern. The carpace 

 may be regarded as consisting of two 

 portions, approximately the posterior fourth 

 being separated from the rest by two very 

 narrow plates which extend across the entire 

 width. In the anterior portion, whose borders 

 are quite smooth, there is an axial row of 

 eleven plates, which, in general, increase in 

 width from before backwards, the fifth and 

 sixth plates being distinguished from the rest 

 by having their lateral margins curved, while 

 in the others they are either straight or angu- 

 lated. A series of six marginal plates, quad- 

 rangular in outline, forms the border of the 

 carapace on each side, a narrow plate, how- 

 ever, lying lateral to the second and third 

 marginals and forming the actual border in 

 that region. Between the marginal and axial 

 plates a number of others are interposed. 

 Anteriorly and corresponding to the second and third axials are 

 two polygonal plates, and posterior to these are three elongated 

 plates of which the most lateral corresponds practically to the 

 second marginal and the innermost to the fourth axial. Corres- 

 ponding to the fifth and sixth axials is a plate almost crescentic in 



Fig. 7. Hansen's y larva. 

 Stage I, from dorsal surface. 

 oc naupliar eye; S., sensory 

 pore. 



