Summaryi. The head of the sea‐urchin spermatozoon is pear‐shaped and axially symmetrical. The sub‐microscopic morphology of the middle piece has not been investigated, but the tail, which terminates in an axial fibre, probably contains spiral or coiled structures, as in mammalian spermatozoa.2. Spermatozoa ofPsammechinus miliarisswim in spirals (frequency, 30–40 per sec), at an average translatory speed of 200/x/sec. at 18oC. This figure applies to very dilute suspensions. The spermatozoa ofEchinus esculentusdo not always swim in spirals.3. The evidence in favour of sea‐urchin spermatozoa being attracted by egg secretions is not conclusive. Experiments to settle this controversial question could be done without great difficulty.4. Little is known about the metabolism of sea‐urchin spermatozoa. The most important features are:(a)No movement in absence of oxygen.(b)Negligible acid production in presence or absence of oxygen.(c)Over certain limits, oxygen consumption per spermatozoon an inverse function of sperm concentration.(d)Cytochromesa, b, c, eanda3, succinic dehydrogenase, cocarboxylase, and catalase (in significant quantities) present.(e)Endogenous substrate unknown, though possibly carbohydrate.(f) Respiration inhibited by cyanide, and photo‐reversibly by carbon monoxide.A large number of substances, of widely different types, are said to stimulate or maintain the oxygen consumption of semen or spermatozoa. Among these are succinate, malonate, iodoacetate, copper, zinc, calcium, albumin and seminal plasma. The mode of action of these substances is in general obscure.5. Sea‐urchin spermatozoa contain a proteolytic enzyme which decreases the viscosity of gelatine, and desoxyribonuclease.6. Sea‐urchin spermatozoa contain or secrete three substances with characteristic chemical and biological properties. These substances and their principal characteristics are:(a)Androgamone I.Not a protein. Inhibits sperm movement in certain circumstances.(b)Androgamone II.Protein. Dissolves or precipitates egg jelly. Also called Antifertilizin.(c)Androgamone III.Not a protein. Liquefies egg cortex. Sometimes called Sperm Lysin.7. The spontaneously reversible agglutination of sea‐urchin spermatozoa in the presence of egg secretions, and in particular of Gynogamone II, has a number of characteristics in common with serological reactions. In recent years this subject has been extensively studied and reviewed by Tyler (1948). Some of the more important conclusions are:(a)Spermatozoa and G. II are multivalent with respect to their complementary combining groups, a condition which is responsible for the agglutination of spermatozoa in G. II.(b)Multivalent G. II can be converted into a univalent form in which it reacts with homologous spermatozoa, but without causing agglutination.(c)The sperm‐G. II reaction exhibits the zone phenomenon (maximum agglutination when the two reactants are present in particular proportions).(d)The sperm‐G. II reaction causes a reduction in the fertilizing capacity of a