88 Part second. 



carries around with her in a so-called brood-pouch. After hatching, 

 all the young develop into males with organs and functions of 

 such, and these same males then change into females. The male 

 and female organs are thus not developed simultaneously as is 

 usually the case in hermaphrodite animals. 



We could specify other Isopods as examples of the degrading 

 influence of this mode of life, and of the depth to which the parasites 

 sink from the hights on which their ancestors stood and their 

 relations still stand. The sessile habit, however, can exercise a 

 similar influence on animals originally free-living, changing them 



out of all recognition. Proof of this 

 is furnished by the Cirripedes (ten- 

 dril-feet), popularly termed Bar- 

 nacles, which are externally so unlike 

 shrimps or crabs, that they have 

 only in recent times been properly 

 understood. Even Cuvier looked 

 upon Balanus, the Acorn-barnacle 

 (fig. 164), and Lepas, the Goose-bar- 

 nacle (fig. 129), as mollusks; and it 

 was not till much later that their 

 Fig. 164. 'Balam^'s perforafus, on ^arly stages and their anatomy re- 

 a rock, 1/2 "at. size. vealcd the fact that they belonged 



to the Crustacea. 

 The general public will therefore also experience some difficulty 

 in accustoming its mind to the fact that these animals are un- 

 doubtedly relations of the well-known species of Crustacea. This 

 may be more intelligible when it is told why we suppose that the 

 curious form of the animal, reminding one of the shell of some 

 fixed mollusk, is due to a far-reaching degeneration. In their 

 early youth, these animals are very small, active and free-swim- 

 ming, with a pear-shaped body and three pairs of swimming-legs. 

 This larval stage is common to all the lower kinds of Crustacea 

 and is termed the "Nauplius" stage. But after several moults 

 this larva fixes itself by its head to some convenient object, and 

 now the skin begins to secrete the calcareous covering, which 

 consists of several plates completely hiding the animal, and only 

 allowing the delicate legs to protrude from a slit-like aperture. 

 These delicate jointed appendages can be seen waving perpetually 

 in both Balanus and Lcpas, serving to create a current bringing 

 food to the mouth (see p. 58). 



Balanus forms a belt all round the rocks just at the surface 

 of the water, and these barnacles are so securely attached that 

 they cannot be washed off by the waves. At low-tide they bear 

 the greatest heat of the sun, lasting out till high-tide with the 

 least drop of water, which they retain in their tightly closed shell. 

 Lepas (when present, in tank No. 22) prefers to attach itself 



