So far as is known, all the Oligochaeta deposit their eggs in special chitinous cases, the cocoons. They share this peculiarity with the Hirudinea. The cocoons have long been known, but were originally mistaken for the eggs themselves. The cocoons contain several eggs and a variable quantity of albumen for the nutrition of the growing embryos. In the majority of earthworms they are more or less oval with projections at the two ends, and are of a brownish colour. In others the tint is rather to be described as green. The genera _Criodrilus_ and _Sparganophilus_ have a cocoon which is greatly elongated. These structures seem to be undoubtedly formed by the clitellum, the earlier {365}opinion of D'Udekem being that they were the product of certain glands developed in _Lumbricus_ at the breeding season, which he thence called the capsulogenous glands. It is more probable that these glands, which have been up to the present but little investigated, are the seat of the formation of the albumen which is found within the cocoons. The cocoons are deposited at varying depths in the ground, or on the surface. Among the aquatic genera they are often attached to aquatic plants. The process of formation has been carefully watched by Vejdovsky[417] in the genus _Rhynchelmis_. The worm throws off the cocoon over its head, crawling backwards to free itself therefrom. The eggs, spermatozoa and albumen, reach the interior of the cocoon as it passes over the orifices of the respective ducts. Out of the numerous eggs which a single cocoon originally contains, only a few, sometimes only one, reaches to maturity. Among the Enchytraeidae, however, quite a large number of young emerge from a single cocoon. The development of all the Oligochaeta is direct, there being no free larval stage. It seems to be the rule for a process of fission to take place in the embryos of _Allolobophora trapezoides_[418] at least, according to the observations of Vejdovsky, in warm weather. In cold weather he found in each cocoon as a rule single embryos, and only 10 per cent of double embryos. [Illustration: FIG. 194.—Cocoons of Lumbricidae. (After Vejdovsky.) A, _Lumbricus rubellus_, nat. size and × 3; B, _Allurus_, nat. size and × 6; C, _Allolobophora foetida_, nat. size and × 3.]