162 THE VOYAGE OF H.M.S. CHALLENGER. 



Furcilia stage (fig. 28) four pairs of gills have made their appearance, of which 

 the anterior three are now bifurcate (see also fig. 26), the last pair simple. In the 

 following stage (first Cyrtopia stage) six pairs may be seen (fig. 29), the three anterior 

 being still bifurcate, whereas the next pair are very small and knob-like ; and of the 

 two last paii's, the anterior are bifurcate, the posterior simple. At the base of the 

 former occurs a rounded expansion, within which a luminous globule has appeared, 

 and hence these two pmirs are found in reality to represent the two last pairs in the 

 adult animal. The antepenultimate pair are of covirse not yet developed, nor is any 

 trace of the corresponding legs as yet perceptible. In the last Cyrtopia stage (fig. 31), 

 however, aU the gills have developed, the four anterior pairs being bifurcate, the fifth 

 simple, the penultimate quadripartite, and the last tripartite. Finally, in the first 

 post-larval stage (fig. 32) the third and fourth pairs have become tripartite, the fifth 

 bifurcate, and the two last pairs respectively quinque- and sex-partite. During the pro- 

 gressive growth of the animal the gills gradually acquire a more complex structure, 

 the two last pairs in j^artieular becoming rapidly larger and more distinctly 

 arborescent. 



Tlie Pleopoda (figs. 33-35). — The development of these limbs does not commence 

 till the Furcilia stage, and proceeds successively from before backwards. In the 

 first of these stages a pair of small bud-like processes are seen springing from the 

 ventral face of the first caudal segment. When isolated and strongly magnified, these 

 processes (fig. 33) exhibit a somewhat lanceolate form, and have on the inner side a very 

 small projection — the first trace of the inner plate. In the stage occurring between the 

 two first Furcilia stages described above, similar bud-like processes successively appear 

 on the following three caudal segments. In the intermediate Furcilia stage, figured in 

 PI. XXIX. fig. 6, the penultimate segment has also acquired a similar pair of buds, and 

 those belonging to the first caudal segment have now attained a more complete develop- 

 ment (fig. 34), being composed of a distinctly defined basal part and two terminal plates, 

 the outer of which is much the larger, and furnished with six natatory set^, whereas the 

 inner is very small, conical in form, and bears a single apical seta only. The other 

 pleopoda now successively acquire a similar structure, while the anterior pair develop 

 further (fig. 35), their inner plate becoming more complete, acquiring at first an 

 additional seta springing from the outer edge, as also a small projection at the inner 

 edge, representing the secondary lobe occurring in the adult animal. The following 

 development proceeds successively from liefore backwards, till finally all the pleopoda 

 have acquired their definite form. 



The Tdson (figs. 3G-42). — This part undergoes a constant change during the several 

 stages of development, being at first very broad and spathulate in form, and becoming 

 successively narrow until at last it assumes the slender spine-like form characteristic of 

 the adult animal. This change is also, as will appear in the sequel, accompanied by a 



