DEVELOPMENT OF EUPHAUSIA SUPERB A 75 



limb has the appearance of having nearly twice as many segments as it actually possesses. 

 The limbs at this stage show a decrease in size and development from in front back- 

 wards. There is also an increase in the number of setae in the larvae having the greater 

 number of pleopods, so that instead of two small setae terminally on the endopod of 

 Th. II and III and one seta on Th. IV and V, as in the larvae with three pairs of non- 

 setose pleopods, there are in the larva with five pairs of pleopods on the endopods of 

 Th. II, III and IV two terminal setae and one or two lateral setae and on the endopod 

 of Th. V one terminal seta. The exopods, with the exception of Th. VI, also bear two 

 or three setae at their tips in the larva with five pairs of pleopods. The exopods 

 themselves in the five-pleopod larva show an advance in development from the small 

 larvae in this group, in that they have begun to lose the "sausage shape" and have 

 commenced to show the first signs of the form found in the adult animal. 



Gills. It is in this stage that the gills begin to be obvious on the thoracic Umbs. In 

 the larva with three pairs of pleopods they are distinguishable as ill-defined saccular 

 processes on the external margin of the limbs Th. II-V. The gill on Th. VI cannot yet 

 be seen. In the krva with five pairs of pleopods the gills of Th. II, III and IV are 

 slightly bilobed ; in Th. V and VI they are simple. Fig. 16 a-d shows the gills in situ ; the 

 bilobed condition is not always obvious when the limb to which the gill is attached is 

 examined dorso-ventrally. There is no sign of a gill on the ist thoracic appendage of any 

 of the larvae examined. 



Pleopods. This stage includes, as stated above, larvae having up to five pairs of non- 

 setose pleopods. In the larva having the smallest number, namely two pairs, the limb is 

 distinctly bilobed, having the external lobe or exopod much longer than the internal 

 (Fig. i6e). In the first pair of limbs there is already a constriction about the middle of 

 the limb where the exopod merges into the protopodal portion. No such constriction is 

 evident in the endopod nor in either the exopod or endopod of the following pair. In the 

 larva with three pairs of pleopods (Fig. 16/) all three pairs are similar to the first pair in 

 the previous larva. There is, however, a decrease in size from in front backwards. In the 

 larvae with four and five pairs of pleopods (Fig. 1 6g, h) the constriction separating exopod 

 from protopodite is well defined and clearly segmented. The endopod still merges into 

 the protopodite. 



Within the integument of these non-setose pleopods there can be distinguished de- 

 veloping setae, and in some the tip of the pleopod is drawn out into small digitiform 

 processes. The importance of the setae within the integument is discussed in another 

 section (p. 38). Occasionally larvae are found in which the pleopods of the last pair are 

 very dissimilar to those that precede them. They are much smaller pear-shaped buds 

 without any constriction at the middle. This form of limb does not show the rudiments 

 of setae within the integument. Infrequently also larvae are found in which, in the last 

 pair of pleopods, the one is more developed than the other. 



Luminous organs. The luminous organ on the ocular peduncle is distinguishable 

 as a fascicle of fibres situated ventrally behind the visual part of the eye. The organ at the 

 base of the limb Th. II is present ; that of Th. VII is not yet apparent, nor are any of the 

 unpaired abdominal ones to be seen. 



