CRUSTACEA. 



413 



FIG. 232. LARVAL FORMS OF THE 

 THORACICA. (From Huxley.) 



A. Nauplius of Balanus balanoides. 

 (After Sp. Bate.) B. Pupa stage of Lepas 

 australis. (After Darwin.) 



n. antennary apodemes ; (. cement 

 gland with duct to antenna. 



antennae, and the cement gland (t) supplies the cementing material 

 for effecting it. A retrogressive metamorphosis of a large number 

 .of the organs sets in, while at the 

 same time the formation of new 

 adult structures is proceeded with. 

 The eyes become gradually lost, 

 but the Nauplius eye is retained, 

 though in a rudimentary state, and 

 the terminal joints of the antennae 

 with their olfactory hairs are thrown 

 off. The bivalve shell is moulted 

 about the same time as the eyes, 

 the skin below it remaining as the 

 mantle. The caudal process be- 

 comes aborted. Underneath the 

 natatory feet, and between the 

 above-mentioned chitinous lamellae, 

 the cirriform feet are formed ; and 

 on their completion the natatory 

 feet become thrown off and replaced 

 by the permanent feet. In the Lepadidae, in which the metamor- 

 phosis of the pupa stages has been most fully studied, the anterior 

 gart of the body with the antennae gradually grows out into an elon- 

 gated stalk, into which pass the ovaries, which are formed during the 

 Cypris stage. At the base of the stalk is the protuberant mouth, the 

 appendages of which soon attain a higher development than in the 

 Cypris stage. At the front part of it a large upper lip becomes formed. 

 Above the mantle and between it and the shell there are formed 

 in the Lepadidae the provisional valves of the shell. These valves 

 are chitinous, and have a fenestrated structure, owing to the chitin 

 being deposited round the margin of the separate epidermis (hypo- 

 dermis) cells. These valves in the Lepadidae " prefigure in shape, 

 size, and direction of growth, the shelly valves to be formed under 

 and around them" (Darwin, No. 519, p. 129). 



Whatever may be the number of valves in the adult the provisional 

 valves never exceed five, viz. the two scuta, the two terga and the carina. 

 They are relatively far smaller than the permanent valves and are therefore 

 separated by considerable membranous intervals. They are often preserved 

 for a long time on the permanent calcareous valves. In the Balanidse the 

 embryonic valves are membranous and do not overlap, but do not present 

 the peculiar fenestrated structure of the primordial valves of the Lepadidae. 



In connection with the moult of the pupa skin, and the conversion 

 of the pupa into the adult form, a remarkable change in the position 

 takes place. The pupa lies with the ventral side parallel to and ad- 

 joining the surface of attachment, while the long axis of the body of 

 the young Cirriped is placed nearly at right angles to the surface of at- 

 tachment. This change is connected with the ecdyses of the antennary 



