THE CIRRI FED I A 123 



istic nauplius structure. In the second and third pairs the exopodite 

 is multiarticulate and beset with natatory setae, and the proto- 

 podite has masticatory hooks. The mouth is overhung by a 

 labrum (01) which in its great size recalls the corresponding organ of 

 the Branchiopod nauplius. It contains a group of gland-cells which 

 open at its tip. The unpaired eye (0') is always well developed, 

 and after the first stage a pair of sensory " frontal filaments " appear. 

 The later nauplius stages are characterised, especially in the Pedun- 

 culata, by the development of spine-like processes from the dorsal 

 shield and by the elongation of the caudal and posterior dorsal 

 spinous processes of the body. A spine which develops from 

 the middle of the dorsal shield was regarded by Dohrn as repre- 

 senting the mid-dorsal spine of the Brachyuran zoe'a. Six pairs of 

 movable spinules are commonly developed in the later stages on 

 the ventral surface of the caudal process, and these have been 

 regarded as corresponding to the rudiments of the six pairs of 

 thoracic limbs. However this may be, the series of nauplius stages 

 is closed with a definite metanauplius stage (Fig. 73, b), in which a 

 rudiment of the fourth pair of appendages (maxillulae) is present. 

 In this stage a downward flexure of the lateral portions of the dorsal 

 shield is observed foreshadowing the development of a bivalve shell 

 which encloses the body in the succeeding Cypris stage. The six 

 pairs of thoracic limbs (F\ F) can be seen developing under the 

 cuticle, as can also the adhesive disc of the antennules and the paired 

 compound eyes. 



At the next moult the larva passes at once into what is knowTi 

 as the Cypi'is stage (Fig. 74). The presence of a large bivalve shell 

 gives it a general resemblance to one of the Ostracoda, but it must 

 be noted that this resemblance does not extend to the number or 

 structure of the limbs nor to the internal anatomy. All the appen- 

 dages of the adult are present, and the larva is now essentially a 

 free-swimming Cirripede. The mouth is closed, and the stage was 

 styled by Darwin a " locomotive pupa." The two valves of the 

 shell are continuous in the mid-dorsal line, and the free ventral 

 margins show a certain asymmetry as in many Ostracoda. An 

 adductor muscle in the same position as in the adult serves to bring 

 the valves together. Near the anterior end on the ventral margin 

 of each valve is a minute aperture which in some cases (Lepas 

 pedinata) is elevated on a blunt horn-like process. In connection 

 with this opening is a gland which is probably to be identified with 

 the gland of the fronto-lateral horn in the nauplius stage. The 

 antennules (Fig. 74, 1) are protruded between the valves of the 

 shell anteriorly. The third segment is expanded into a sucker-like 

 disc, on which opens the duct of the cement-gland, and which serves 

 for the occasional temporary attachment of the larva. The terminal 

 segment is attached to the side of the third, and bears besides 



