66 DISCOVERY REPORTS 



group of larvae which has seven terminal spines on the telson — all later larvae (and 

 most of the seven-spined larvae) have the three-segmented form of palp. 



Inspection of Table XX shows that larvae can occur having the fifth pair of pleopods 

 non-setose but with the mandibular palp in an advanced stage of segmentation (Note i). 



The majority of larvae which are going to moult again into the second form of seven- 

 spined telson larvae have mandibular palps which are primitive in form, but this is not 

 invariable. 



The numbers of segments noted in the mandibular palp, and observations of develop- 

 ment within the existing integument, indicate that the general process of development 

 is from one to three segments, although occasional larvae have a two-segmented form. 



In Table XXVII the length frequency of the larvae and the number of spines in the 

 mandibular palp are stated. In addition each group of larvae distinguished by the 

 number of spines on the mandibular palp has been divided according to the number of 

 terminal spines on the telson. As in the segmentation of the antennal endopod, so in the 

 mandibular palp, increasing length is accompanied by increase in the number of spines 

 on the palp, but the length range of larvae for any particular spine number overlaps in 

 its upper and lower limits the ranges which precede and succeed it. 



Larvae with seven terminal spines may have no spines or one or two spines on the 

 mandibular palp, those with five telson spines may have one, two, three or four spines 

 on the palp and so on, so that no mandibular palp spine number or range of numbers is 

 exclusive to larvae having a particular telson spine number. The general development of 

 course is such that the number of spines on the mandibular palp is increased with de- 

 crease in the number of spines on the telson, and, as in the segmentation of the antennal 

 endopod, within any one group distinguishable by the number of telson spines, increase 

 in the number of palp spines is directly connected with the average length of larvae. 



First thoracic appendage. In the Calyptopis and earlier Furcilia, the ist thoracic 

 limb is as shown in Fig. 6, having two basal segments, an exopod consisting of a single 

 segment and an endopod of two segments. As already demonstrated with larvae that 

 have five pairs of setose pleopods and still have the unaltered number of seven terminal 

 spines on the telson, the ist thoracic limb changes from its primitive condition and the 

 endopod assumes its final form by an increase in segmentation from two to five. 



Examination of the data obtained from the larvae from St. 374 indicates that this 

 change invariably takes place before the telson becomes five-spined. In the seven- 

 spined larvae the primitive form of the ist thoracic limb is confined to the smaller larvae 

 which are going to moult again into a seven-spined form. 



Rostral spine. The rostrum of each larva was examined to find the frequency of 

 occurrence of the median spine which foreshadows the pointed condition of the adult 

 animal. It is seen that in all forms of larvae distinguished by telson spine number the 

 pointed condition can occur, and that even in larvae having only one terminal spine on 

 the telson it is possible to have the rostral spine undeveloped. The number of larvae 

 which have a rostral spine does, however, increase appreciably as the larvae develop. 

 This is shown in Table XXVIII, where the percentages of the total for each group are 



