74 THE BIOLOGY OF MARINE ANIMALS 



tubes. Because of their importance these structures are described in more 

 detail in a separate chapter (15). We note here the occurrence of CaC0 3 

 in coral skeletons and molluscan shells, and of CaC0 3 and Ca 3 (P0 4 ) 2 in 

 the exoskeleton of decapod crustaceans. Magnesium is an important 

 constituent of the skeleton of certain animals — Foraminifera, Alcyonaria, 

 Echinodermata and Crustacea. Magnesium concentrations in some gastro- 

 pods are very high (1-58% wet weight in Archidoris). Strontium occurs in 

 the skeleton of some radiolarians (Acantharia). Silicon is important in the 

 skeleton of diatoms, most radiolarians and siliceous sponges. The radular 

 teeth of limpets (Patella) contain much silica (33 % of ash), as well as 

 iron (56, 141). 



EGGS, EMBRYOS AND LARVAE 



Eggs of marine invertebrates are usually in osmotic equilibrium with the 

 surrounding sea water, but differ considerably from the latter in ionic 

 composition (Table 2.8). Echinoderm eggs have been extensively studied, 

 and over a limited range of dilutions they behave in conformity with the 

 gas laws. The plasma membrane is largely impermeable to salts and the 

 egg behaves like an osmometer when placed in dilute sea water. Deviations 

 from expected values for volume changes are explained as due to the 

 presence of osmotically inactive materials. The osmotically inactive frac- 

 tion amounts to 7-3% in unfertilized eggs of Arbacia, and increases to 

 27-4% after fertilization. 



Cyclical changes take place in the egg prior to and subsequent to fertiliza- 

 tion, one of which is permeability to water. Sensitivity to dilute sea water 

 and rate of swelling in hypotonic media increase greatly after fertilization. 

 The permeability of the egg membrane (Arbacia, Paracentrotus) alters 

 greatly at this time: potassium, calcium and magnesium ions are released 

 shortly after fertilization and are subsequently resorbed (95, 122). 



A proportion of the total osmotic pressure of eggs is due to organic 

 molecules, particularly in higher forms. Relative amounts of organic 

 constituents are low in echinoderm eggs, whereas in eggs of Maia, Sepia 

 and Torpedo they are responsible for one-quarter to one-half of the total 

 osmotic concentration. Among mineral constituents potassium is high, as 

 in many cells, sodium occurs in relatively low concentrations, calcium and 

 magnesium are present in colloidal combinations and chloride is mostly 

 dissociated, the relative amounts varying in different species. To maintain 

 these ionic differences from the environmental medium requires a highly 

 impermeable barrier about the egg. In reality, the egg is selectively perme- 

 able, and possesses the power of secreting and absorbing particular ions 

 according to metabolic requirements. During the course of development 

 salt is absorbed, and the ash content and density steadily increase (84, 94). 



Ionic regulation in developmental stages of marine animals has been 

 reviewed by Krogh (77), and Needham (94) has collated much relevant 

 information. An interesting study of an estuarine polychaete Marphysa 

 gravelyi reveals that the demersal eggs of this species are protected by a 



