LIFE-HISTORY OF THE ROCK BARNACLE. 83 



In Balanus there are altogether exactly six stages, and the 

 most ready way to distinguish these is by measurement. Figs. 11 — 

 16 show the form of the carapace at each stage, and at the same 

 time the relative size, and certain other characters. We see, then, 

 that the last Nauplius stage is between three and four times the 

 length of the first, and whereas the newly hatched Nauplius is only 

 just visible to the naked eye, the oldest are easily recognisable. 

 During this growth highly important changes take place in both 

 internal and external organization. These lead up to the last larval 

 condition known as the Pupa or Cypris-stage. The Nauplii of 

 the sixth stage (Fig. 22) are beautiful and instructive creatures. 

 One can see already the compound eye of the Cypris-stage, at first 

 red and showing the retinulae (Fig. 23), later black ; the six pairs of 

 thoracic legs are visible but still lie within the cuticle, and can be 

 seen to take their origin in a series of paired spines on the ventral 

 side of the larva. 



An exceedingly interesting feature about the Nauplii of some 

 Cirripedes is the effect of light on the direction of their movements. 

 At some times the Nauplii move towards the light, and at others 

 away from it. In one species (Balanus perforatus) upon which the 

 author, in conjuction with Dr. Loeb has experimented, the Nauplii 

 performed daily migrations in the vessels ; in the early morning they 

 commonly sought the light, while a little later on and during the 

 greater part of the day they avoided it. Beautiful experiments can 

 be made on these little creatures, they can, for instance, be made to 

 follow a candle all round a glass vessel. There can be little doubt 

 that we have here a partial explanation of the daily oscillations 

 performed by a great number of the animals living at the surface of 

 the sea. When they have been sufficiently long exposed to the 

 light, they retire to the depths. Whether this is done for the 

 purpose of concealment or for some other object, it is difficult to say. 



The Cypris-stag"e or last larval condition, is another exceedingly 

 interesting form. The larva attains this stage, so utterly different in 

 appearance from the Nauplius, in a single moult. In some forms, the 

 Cypris-stage immediately after the moult shows curious transitional 

 features between it and the Nauplius (Figs. 24 & 25) ; these inter- 

 mediate forms are important for the understanding of the structure of 

 the fully formed Pupa. In the latter (Figs. 26 — 28) the shield-shaped 

 carapace has assumed the form of a bivalved shell, open below for most 

 of its extent. The caudal spine and the horns are quite lost, and the 

 antennules have suddenly become transformed into a most remark- 

 able pair of prehensile organs of curious shape (a 1 ), provided with a 

 complex series of muscles (Fig. 27) to enable them to perform a 

 variety of movements. They are usually seen completely withdrawn 



