. The Cambridge natural history. Zoology. 6o2 ECHINODERMATA as for instance in Hemiaster philippi (Figs. 250, 281), the eggs are carried in some of the deeply grooved petaloid ambulacra; whilst in Holothuroidea they may develop in the body-cavity {Phyllophorus urna), or they may adhere to the back of the mother {Cucumaria crocea, Fig. 259, p. 573), or they may be protected in special brood-pouches either on the ventral side of the parent (Cucumaria laevigata) or on the dorsal surface (Fsolus ephippifer, Fig. 261). The majority of these cases of embryonic development have been recorded from Arc
. The Cambridge natural history. Zoology. 6o2 ECHINODERMATA as for instance in Hemiaster philippi (Figs. 250, 281), the eggs are carried in some of the deeply grooved petaloid ambulacra; whilst in Holothuroidea they may develop in the body-cavity {Phyllophorus urna), or they may adhere to the back of the mother {Cucumaria crocea, Fig. 259, p. 573), or they may be protected in special brood-pouches either on the ventral side of the parent (Cucumaria laevigata) or on the dorsal surface (Fsolus ephippifer, Fig. 261). The majority of these cases of embryonic development have been recorded from Arctic or Antarctic waters ; it appears as if conditions there were not favourable to the larval type of development. In Pelmatozoa the develop- ment of Antedon rosacea alone is known, and that is of the embryonic type. So far, however, as their mode of propaga- tion is known, it may confidently be affirmed that the development of the majority of the Fig. 280.—Oral view of Asterias spirahilis, slightly g-nggjeg of Eleutherozoa enlarged, showing embryos attached to the everted . « , lips. cmJi, Embryos. (After Perrier.) IS of the SeCOUd Or larval type. In this type there is little food-yolk in the egg, and the young animal or larva is forced from a very early period of development to seek its own living, and hence it is usually a consider- able time (from a fortnight to two months) before the adult form is attained. When the emlryos of different groups of Eleutherozoa are compared, there is no obvious agreement in structure between them; but the larvae of the four classes of Eleutherozoa exhibit with differences in detail a most remarkable fundamental similarity in type, and we are accordingly justified in regarding the larval development as primitive, and the embryonic type as derived from it and differently modified in each case. In the typical larval development the eggs are fertilised. Please note that these images are extracted from scanned page images that may have been di
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
