Eggs of Crustacea.—Theof the cs tacea are notable as containing en ru larger quantity of yolk in proportion to their size than do those of any other animal. In nearly all species the eggs are carried around by the female during the early stages of their development, either attached to some portion of her body or in a brood pouch. They do not always hatch into the complete form, but may advance, like many of the crabs, by changes with each moulting of the shell to the final adult form. With this large measure of protection the number of eggs laid as a °brood° is generally few.
Eggs of As a rule the mollusks are oviparous, but there is a considerable num ber, among them many of the land and fresh water snails, which are ovoviviparous. The number of eggs laid at a time varies greatly. Helix aspersa lays from 40 to 100 in a little de pression hollowed out between the roots of a tuft of grass and covers them with earth. The oyster lays from 300,000 when young to 60,000, 000 when a full-grown adult. These eggs are almost microscopic in size, and they are de voured in thousands by fish and other marine animals. Some of •the tropical mollusks lay eggs as large as those of pigeons, but with a °shell° of tough membrane without a lime de posit to harden them. One of the peculiarities of egg production among the mollusks is their fashion of laying them in small clusters in cap sules or egg-pods. These capsules are usually joined together in curious ways by fibrous bands or strings or ribbons, which sometimes are three feet in length and in others are an inextricably tangled snarl. One of the most curious of all is the cochlostyla of the Philip pines, which lays its eggs in a folded leaf at the tops of the highest forest trees. For an intimate study of this most interesting section of animal life the books mentioned below are of advantage.
Eggs of Lower Among the higher worms may be mentioned the leech, which burrows into the soft mud above the level of the edge of the pool in which it lives and lays several eggs with yolks in a capsule.
The tiny leeches upon hatching find their way instinctively out of the burrow and into the water. The tapeworm develops a clutch of eggs in each of the lowerjoints of its long body. As the eggs approach the larval stage the joints break loose and become simply egg pods, similar to those of the mollusks. Many of the lo'wer worms lav eggs which are rela tively large and are protected by an enveloping capsule. The starfish and sea-urchin pass their .eggs out through pores at the base of the arms or spines, and there they are held in small hol lows between the armor plates until they are so far developed that they push themselves loose of their own accord and swim off independ ently. The egg production of the crinoids is similar, the eggs being retained within the body cavity until they have reached the larval form, when they are released. The ophiuroids carry their eggs about with them in a pouch, where the young develop. In the sponge the egg forms in a cell within the body and undergoes considerable development, so that at the time when it is thrown off by the parent body it has become a larva capable of independent exist ence.
Bibliography.— Calman, W. T., (Crustacea) (London 1909) ; Comstock, J. H.,