212 



CRUSTACEA. 



The Nauj>lii of the Lepadidae, when hatched, seem closely to 

 resemble tlie Naiqilii of the Bcdanidae in shape, differing only in 

 this one respect, that the long frontal horns are bent backwards. 

 Soon after hatching they undergo a first moult, after which the 

 spine-like thoraco-abdominal appendage (ventral spine) and the dorsal 

 caudal spine first attain their full length. During later moults, the 

 NaupUus grows to a great size (12 mm. long), and is then distinguished 

 by a number of accessory structures. In Lepas fascicularis, seven 

 moults take place before the Metanaupilius stage is reached. These 

 later stages (Fig. 103) show the thoraco-abdominal appendage (t), 

 and the dorsal caudal spine (ds) drawn out as long, pointed processes 

 beset with small hooks. A similar process (d) has arisen at the 



central point of the dorsal 

 shell ; this process, which runs 

 obliquely backwards, is known 

 as the dorsal spine. At the 

 margin of the dorsal shield 

 also, besides the frontal horns, 

 eight large spine-like processes 

 can be recognised, the first pair 

 lying anteriorly between the 

 frontal horns, and the others 

 being distributed laterally and 

 posteriorly. Both the dorsal 

 shield and the larger processes 

 are further beset with a series 

 of minute spines, which are 

 wanting only on the frontal 

 horns. Many of these minute 

 spines, as well as the marginal 

 spines of the dorsal shield, are 

 found to be jierforated and 

 connected with the ducts of 

 unicellular integumental glands. 

 On the ventral side of the 

 thoraco - abdominal appendage, 

 two more posteriorly-placed, immovable thorns can be distinguished,, 

 and, anteriorly, six pairs of larger movable spines (x), which do 

 not appear simultaneously but successively, each new moult develop- 

 ing a fresh pair, until, after the last moult, the full number of 

 six is attained. The conjecture, made with some hesitation by 



Fig. 103. — Larva of Ltims cmstralis (Archizoaea 

 gigas) (after Dohrn). a, anal aperture ; d, 

 dorsal spine; ds, dorsal caudal spine; h, 

 frontal horns ; ol, upper lip ; t, thoraco- 

 abdominal appendage (yentral spine); x, 

 paired movable spines. 



