246 ARTHROPODS. 



drawn out into a long flexible stalk, containing a cementing gland, the 

 ovaries, etc., and involving in its formation the first pair of antennfe and 

 the front lobe of the head. The mouth region bears a pair of small 

 mandibles and two pairs of small maxillae, the last pair united into a lower 

 lip. The thorax has six pairs of two-branched appendages, and froni 

 the abdomen a long penis projects. Around the body there is a fold of 

 skin, and from this arise five calcareous plates, an unpaired dorsal 

 carina, two scuta right and left anteriorly, two terga posteriorly. The 

 nervous system is fairly complete, but there are no eyes. There is a 

 complete food-canal and a large digestive gland. Beside the latter he 

 the lobes of the testis, from which a vas deferens runs to the penis. 

 The oviducts from the ovaries in the stalk are said to open at the base of 

 the anterior thoracic limbs, and the eggs are found in flat cakes between 

 the external fold of skin and the body. 



The life-history is most interesting. Nauplius larvse escape from the 

 egg-cases, and after moulting several limes become like little Cyprid 

 water-fleas. The first pair of appendages become suctorial, and after a 

 period of free-swimming, the young barnacle settles down on some 

 floating object, mooring itself by means of the antennary suckers, and 

 becoming firmly glued by the secretion of the cement glands. Then im- 

 portant changes occur, the valved shell is developed, and the adult 

 form is gradually assumed. While the early naturalists, such as 

 Gerard (1597), regarded the barnacle as somehow connected with the 

 barnacle-goose, and zoologists, before J. Vaughan Thompson's re- 

 searches (1829), were satisfied with calling Cirripedes divergent 

 Molluscs, we now know clearly that they are somewhat degenerate 

 Crustaceans. We do not know, however, by what constitutional vice, 

 by what fatigue after the exertions of adolescence, they are forced to 

 settle down to sedentary life. 



The food consists of small animals, which are swept to the mouth by 

 the waving of the curled legs. Growth is somewhat rapid, but the 

 usual skin-casting is much restricted except in one genus. Neither the 

 valves, nor the uniting membranes, nor the envelope of the stalk are 

 moulted, though disintegrated portions may be removed in flakes and 

 renewed by fresh formations. An allied barnacle, Scalpellum ornatum, 

 has separate sexes, and another species of this genus, Scalpellum vulgare, 

 has a male which is not like a Cirriped, a fact which may be interpreted 

 as an illustration of the superior vigour of the sex. 



Balanus, the acorn-shell, encrusts the rocks in great numbers between 

 high and low water marks. It may be described, in Huxley's graphic 

 words, as a crustacean fixed by its head, and kicking its food into its 

 mouth with its legs. The body is surrounded, as in Lepas, by a fold of 

 skin, which forms a rampart of six or more calcareous plates, and a 

 fourfold Ud consisting of two scuta and two terga. When covered by 

 the tide the animal protrudes and retracts between the valves of the 

 shell six pairs of curl-like double thoracic legs. The structure of the 

 acorn-shell is in the main like that of the barnacle, but there is 

 no stalk. 



The life-history also is similar. A Nauplius is hatched. It has the 

 usual three pairs of legs, an unpaired eye, and a delicate dorsal shield. 

 It moults several times, grows larger, and acquires a firmer shield, a 



