256 SCIENCE PROGRESS 



oneself of this fact by comparing, in imagination, a series of 

 cross-sections through a number of the most familiar shore 

 forms, such as crab, limpet, barnacle, starfish, brittlestar, 

 sand-hopper, sea-slater, butter-fish, flounder, and so on. In 

 some the flattening is lateral, in others dorso-ventral. We should 

 also call to mind the numerous encrusting forms, such as the 

 " crumb-of-bread " sponge, various polyzoa, the bivalve Ano- 

 mia, sea-squirts like Styleopsis. It is not meant to suggest 

 that flattened forms do not occur elsewhere, but rather that 

 they are particularly characteristic of the shore. The so-called 

 " stream line " form is conspicuous by its absence. This is 

 because the water is not acting in one determinate direction, as 

 in rivers, but all round. The most typical shape is that of the 

 limpet, and even so there is evidence for believing that limpets 

 which live on exposed rocks have lower spires than those living 

 in sheltered situations. 



The danger of being swept away which is ever confronting 

 the adult shore animal menaces still more strongly its eggs 

 and young. No other vital function reflects more clearly the 

 difficulties of the environment than that of reproduction, 

 and it is consequently not surprising to find that reproductive 

 phenomena in the area of wave-action are particularly interest- 

 ing. In the first place, shore animals are frequently very 

 prolific ; the egg-ribbon of a sea-slug may contain sometimes 

 not much less than a million eggs. This is one method. As 

 an alternative, only relatively few eggs may be deposited, but 

 these are made the object of special care on the part of the parent. 

 The parent gunnel {Centronotus gunnellus) coils itself round 

 the eggs and protects them with its body. Another rock-fish, 

 the lump-sucker {Cyclopterus lumpus) fans the eggs with its 

 tail to ensure their constant aeration. The stickleback builds 

 a nest. Other forms hatch the eggs inside the body, or, in 

 other words, are viviparous. Fertilisation is frequently inter- 

 nal, and the eggs, when deposited, are enclosed in a protective 

 envelope, which adheres to the substratum. This may be 

 gelatinous, as in some Polychaets and most sea-slugs, or 

 chitinous, as in the whelk {Buccinum undatum) and dog- whelk 

 {Purpura lapillus). 



We have seen that there is a sense in which the rise and fall 

 of the tides has provided an opportunity for marine animals 

 to attempt the conquest of the land. Looked at from another 

 view-point, the alternation of aquatic and land conditions 

 presents a very urgent problem, or set of problems. Marine 

 animals require to breathe dissolved oxygen, and are unable 

 to utilise directly the oxygen of the atmosphere. Further, the 

 bodies of many, though by no means of all, marine animals, 

 are soft, contain a great deal of water, and are unable to resist 



