First lesson in zoology : adapted for use in schools . mentalvertebrate marks, that of a rudimentary backbone, , anotocord, with the nervous system placed above, and theother internal organs below. But there is a group of animals which partly bridge overthe gap between the lancelet and the worms. These are the sea-squirts or ascidians (Fig. 147). Oneof them, called Appendicularin, is like a tadpole in generalappearance, while the larvae of most ofthem are tadpole-shaped, as in Fig. 148. Inthe infant ascidian the tail is supported bya gristly rod {u), extending into the chest,and correspond
First lesson in zoology : adapted for use in schools . mentalvertebrate marks, that of a rudimentary backbone, , anotocord, with the nervous system placed above, and theother internal organs below. But there is a group of animals which partly bridge overthe gap between the lancelet and the worms. These are the sea-squirts or ascidians (Fig. 147). Oneof them, called Appendicularin, is like a tadpole in generalappearance, while the larvae of most ofthem are tadpole-shaped, as in Fig. 148. Inthe infant ascidian the tail is supported bya gristly rod {u), extending into the chest,and corresponding to the notocord of thelancelet. Above it is the nervous cord («),and in the head is an eye {(>) and an ear (o),* AnascSancS ^l^ile the mcuth (.-) opens into a pharynx,uraisize) which in after-life has gill-slits. Towards adult life the ascidians (perhaps except Appendicularia)take a backward path in their development, and lose thestartling vertebrate features of their youth. Like someprecocious human children, they cease to fulfil the promise. THE SKVLLBSS VERTEBRATES. 141 of tlieir youth, and go downward and backward, becomingscarcely higher than clams. But without doubt some fa-vored forms, now lost to science, pressed onward and up-ward, held on to their tails, preserved their notocord, be-came active swimmers, dashed vigorously about after other
Size: 1385px × 1804px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1894
