326 LECTURE XV. 



covered by the shell, and opening exteriorly, one by the side of 

 the other, at the fore-part of the extremity of the abdomen, 

 where they communicate with the canal formed by the incurved tail. 

 The first two-thirds of the ovarium lies free in a capsule, formed 

 by the membrane which lines the valves. The ova are minute at 

 the blind beginning, but progressively augment in size as they 

 approach the outlet, or the fixed vertical and terminal part of the 

 oviduct. These little Entomostraca oviposit on some fixed foreign 

 body, agglutinating their ova in a mass of some hundreds derived 

 from several individuals, by a substance of a greenish filamentary 

 structure like moss. Thus, like the ostrich, many females con- 

 tribute to one nest. In four days and a half the eggs are hatched ; 

 the young come out in form like the parent ; there is no metamor- 

 phosis, only the valves are of a slightly different form, and of a red 

 colour. As the pools dry, they bury themselves in the sand or mud 

 at the bottom ; if that remains moist, the old animals survive ; if it 

 becomes dry, they perish, but their eggs retain their latent life, and 

 are hatched on the return of rain. This explains the seemingly 

 marvellous phenomenon sometimes observed in a shallow pool, which 

 has been dried up in the summer, and may have long remained so, 

 but which, in a day or two after it has been refilled with water, is 

 found swarming withjittle Entomostraca. 



In the Branchipus stagnalis we have another example of the 

 rapidity of the appearance of swarms of individuals in recent pools 

 through a similar economy of latent vitality of the ova. The Bran- 

 chipus is characterised by the great number of its segments ; the 

 males are distinguished by their antennae having long antler-like 

 terminal joints or processes, which serve to retain the female ; and at 

 the termination of the sperm-ducts are two intromittent plates. There 

 are two sperm-sacs and two testes in the form of long and straight 

 caeca, which extend the whole length of the tail ; and from the 

 anterior dilated end the .sperm-duct proceeds inwards and back- 

 wards. Both sperm-ducts soon after their origin dilate into a sperm- 

 sac; and then proceed to a swelling at the base of the tail, where 

 they open near a spine-bearing peduncle. The ova, when they are 

 impregnated, are received into an external capsule. When the pools 

 are dry, the ova have the power of retaining life for two or three 

 years. 



The generative organs attain an enormous extent in Limulus : 

 they are packed closely, interblending with the ramified caeca of 

 the liver, in both male and female, throughout the body. In the 

 female the ovarian tubes present the dendritic or ramified dispo- 

 sition. We distinguish the sexes by the external character of 



