§4 DISCOVERY REPORTS 



setose have increased in size and show the beginnings of a third lobe at the base of the 

 inner lobe (Fig. 19 c). The gill of Th. VII has already been described in connection with 

 the limb rudiment. There is no sign in this Furcilia stage of a gill on the ist thoracic 

 limb. 



Pleopods. The variety of pleopod forms within this stage is indicated in the list at the 

 beginning of this section. The appearance of the pleopods in larvae having respectively 

 three, four and five pairs setose is shown in Fig. ig d, e, f. The non-setose pleopods 

 are typically as in the larger larvae in the preceding stage, that is, having the exopod 

 marked off' from the protopodite and the endopod merging into it. Within the integu- 

 ment of the non-setose pleopods there can be distinguished the setae with which the 

 exopod will be furnished when the larva moults (Fig. 19 h). The setose pleopods have 

 six to eight setae on the exopod, which is now divided off from the protopodite, and 

 from one to three setae on the endopod. At this stage the appendix interna appears on 

 the inner margin of the endopods of those pleopods which are setose (Fig. 19 ^). In the 

 smaller larvae it is represented by a small hook-like process on the endopod, whilst in 

 the larva with five setose pleopods a number of these hooks are borne on a digitiform 

 projection from the inner margin of the endopod. 



Luminous organs. In addition to those on the ocular peduncle and at the base of 

 Th. II there are the following unpaired luminous organs distinguishable between the 

 pleopods : 



The larva having 



2 setose pleopods and 3 non-setose has luminous organs on abdominal segments i and 2. 



3 setose pleopods and 2 non-setose has luminous organs on abdominal segments i, 2 and 3. 



4 setose pleopods and i non-setose has luminous organs on abdominal segments i, 2, 3 and 4. 



5 setose pleopods has also four abdominal luminous organs. 



The development of the luminous organs on the abdominal segments thus coincides 

 with the development of setae on the pleopods. 



The luminous organ on Th. VII cannot yet be distinguished. 



THIRD FURCILIA 



The reasons for recognizing this form of larva as a separate stage are given above on 

 pp. 41-9. The frequency of occurrence of these larvae and their average lengths at dif- 

 ferent stations are stated in Table XXXI. For all the 3rd Furcilia the average length is 

 7-32 mm., and the range lies between 579 and 875 mm. 



The two previous Furcilia stages are recognizable by the number and degree of de- 

 velopment of the pleopods. In this stage other criteria must be adopted, as the pleopods 

 have reached their full number and are all setose as in the largest larvae of the preceding 

 stage. The most convenient character for this purpose is the form and number of the 

 telson spines ; the number is the same as in the earlier Furcilia stages, but the innermost 

 postero-lateral spine, as shown in Fig. 20 c, is altered, being greatly widened at the base. 

 Other distinctions in development in this Furcilia stage will be mentioned below. 



