THORACICA. 219 



In Anelasma, the substratum is affected by simple pressure, but in the other 

 forms just mentioned by a boring action of the stalk. 



The passage of the stalk out of the Cypris shell is rendered possible by the 

 fact that the latter at this stage surrounds the body very loosely. Whereas, in 

 the Cypris stage, the Avhole of the ceplialic region is contained within the bivalve 

 shell, in the adult, the anterior part of that section (the stalk) is not covered by 

 the shell, here, as in the Cladocera, only that part of the head which carries the 

 mouth-parts being included within the shell. We may imagine this change to 

 have arisen at the time when the stalk grew out by a secondary flattening out of 

 that anterior part of the mantle-fold which, in the Cirripede pupa, covered the 

 most anterior part of the head. 



The adult Cirripede shell now appears more distinctly beneath the 

 Cypris shell, and in it can be made out the first rudiments (primary 

 valves, Darwin) of the five calcareous plates (scuta, terga, and carina, 

 Fig. 106 C, sc, f, c). These primary valves are distinguished by 

 their sieve-like sculpturing, which is caused by the boundaries of the 

 matrix-cells remaining evident in the calcareous secretion. The 

 surfaces of the valves are covered by a thin cuticle. The primary 

 valves do not directly increase in size, but new calcareous layers 

 are continually being secreted below them, and these attain a 

 size greater than that of the original valves. In a superficial view 

 of the primary valves, they are now seen to be surrounded by 

 concentric lines representing the subjacent calcareous layers. By 

 this method of increase of the valves, the non-calcareous parts of 

 the shell which spread out between the valves become more and 

 more circumscribed, but in some cases these intermediate spaces are 

 retained to a considerable extent (^Conchoderma). It should be 

 mentioned that in those forms which have a large number of 

 valves, only the five primary valves are at first formed. 



The metamorphosis of the Balanidae, in the first stages, resembles that of the 

 Lepadidae. Here also the Cypris pupa gives rise to a young form attached by a 

 short fleshy stalk. Only later does the broad base characteristic of the Balanidae 

 develop and form that external secondary mantle-fold, within which the upper 

 part of the mantle which carries the scuta and terga appears like an operculum. 

 The first rudiments of the shell are here membranous, and the sculpturing 

 mentioned above, which is characteristic of the Lepadidae, is wanting. 



While the external form of the adult is reached in this manner, the inner 

 organs are also undergoing important changes, some of which are as yet little 

 iinderstood. Some organs are cast off at ecdysis (paired eyes, antennal apodemes), 

 others simply degenerate (antennal muscles). Meantime the hepatic outgrowths 

 appear in the intestinal canal. The cement-gland increases considerably in size, 

 and the genital organs show special development. The ovaries undergo their 

 characteristic displacement, shifting to a position within the stalk. 



The moult which follows, and in which the cuticle of the Cypris stage is cast, 

 closes this period of development. In the moult, the outer cuticle of the two 



