DEVELOPMENT OF EUPHAUSIA SUPERB A 41 



velopment of any arthropod, since the course of development is in all cases marked off 

 into stages by the occurrence of ecdyses between which no marked change in form 

 usually occurs. Where ecdyses are numerous and the morphological changes occurring 

 at each are slight, the development is of the type to which the term "continuous" has 

 been applied. Where ecdyses are reduced in number while the total change to be achieved 

 remains the same, the amount of change at each moult will obviously increase till it may 

 reach a degree deserving the name "metamorphosis". 



Larval specializations requiring elaboration of the straightforward course of develop- 

 ment may further increase the extent of change that has to be accomplished at a single 

 moult. 



There is a further consideration to which attention may here be drawn. In the more 

 specialized forms with fewer ecdyses the position of each moult on the scale of develop- 

 ment, and therefore the changes occurring at each, becomes as it were standardized. 

 In the more primitive cases with so-called "continuous" development the position of 

 the moult may vary, and the changes effected at each are not always identical. The series 

 of moults may, in fact, be regarded as a kind of "grid" superposed on a course of 

 actually continuous development. In the more primitive cases this grid may still 

 shift slightly backwards and forwards; in the more specialized cases the "grid" has 

 become fixed and all individuals show the same changes at each moult. 



INTERMEDIATE FURCILIA STAGES 

 In Table XII above it is indicated that there are two different forms of larvae having 

 all pleopods setose and having seven terminal spines. Examination of the appendages 

 of these Furcilia shows that there is a very great amount of variation in the degree of de- 

 velopment. This is most noticeable in the mandibular palp and ist thoracic limb. There 

 are also differences in size in the antennular fiagella, in the postero-lateral spines of the 

 telson and in the shape of the rostrum and telson. 



Nine selected larvae of differing sizes from one sample were examined to discover the 

 range of variation occurring in the mandibular palp, ist thoracic limb and telson. It was 

 seen that the mandibular palp varied between the small bud-like process typical of 

 earlier stages and an elongated three-segmented setose appendage not unlike that found 

 in the adult. The endopod of the ist thoracic limb could be from twice to three times as 

 long as the exopod and be composed of from two to five segments. The innermost 

 postero-lateral spine on the telson varied in the breadth of the proximal portion ; in the 

 smaller larvae the base of the spine was narrow, in the larger ones it was broadened. 



It is possible in some of the larvae to determine the number of terminal spines which 

 are to be present when the larva moults (cf. Macdonald, 1928, pi. iv, fig. 10) and it is 

 found that some are to be seven-spined and others five-spined. Before ecdysis the soft 

 tissues draw away from the enclosing integument, and in the telson region particularly 

 the form of the succeeding stage is defined. It cannot be stated for certain in all the 

 larvae, but in those where it is possible the length frequency distribution correlated with 

 the number of spines in the succeeding stage indicates that the smaller forms clearly 



