. Outlines of zoology. Zoology. 408 UROCHORDA OR TUNIC ATA. On the other hand, the development exhibits marked alternation of generations, both solitary and colonial forms being included in one life history. In Doliolum the fertilised egg gives rise to a tailed larva, which develops into an asexual "nurse, possessing the power of budding (cf. Compound Ascidians). The ventral stolon of the nurse gives rise to a number of primitive buds, which migrate over the body until they reach a dorsal outgrowth, apparently well supplied with blood. Here they fix themselves and divide up to form three
. Outlines of zoology. Zoology. 408 UROCHORDA OR TUNIC ATA. On the other hand, the development exhibits marked alternation of generations, both solitary and colonial forms being included in one life history. In Doliolum the fertilised egg gives rise to a tailed larva, which develops into an asexual "nurse, possessing the power of budding (cf. Compound Ascidians). The ventral stolon of the nurse gives rise to a number of primitive buds, which migrate over the body until they reach a dorsal outgrowth, apparently well supplied with blood. Here they fix themselves and divide up to form three series of buds—two lateral. Fig. 176c.—Anatomy ot Afpendicularia.—After Herdman. , Sense organ ; br., branchial aperture ; at., dorsal tubercle ; ot., otocyst; , nerve ganglion;^., peripharyngeal band; n., nerve cord; 02., oesophagus; St., stomach; ov., ovary; tes., testes; z., intestine; h., heart; u., urochord, cut at «'; , ;, nerve ganglia of tail; 7K., muscle band of tail; app., tail cut through ; a., anus ; at., one of the atrial apertures. and one median. All these buds develop into individuals belonging to the sexual generation, but a few only become truly sexual. The two lateral series develop into nutritive forms, which supply the nurse with food. The nurse itself loses its alimentary and respiratory organs, and becomes a mere organ of locomotion. The median buds develop into "foster mothers," which ultimately go free, bearing with them other buds destined to develop into the solitary sexual forms. In these, first ova and then spermatozoa are produced, which start the life-cycle afresh. It is thus obvious that there is considerable division of labour in the sexual form, accompanied by polymorphism ; the whole process presents some curious analogies to the conditions seen in the Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appe
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