. The Cambridge natural history. Zoology. 42 ASCIDIANS loop gives rise to the double or paired " vessels " seen branching through the test (see Fig. 17, v). The two vessels of a pair are â one blood-channel imperfectly divided by a connective-tissue septum. The l)lood courses out along one side, round the com- munication in a " terminal knob " at the end, and back down the other side. The " terminal knol)S " are very numerous, and form a marked feature in the outer layer of the test (Fig. 17, tJc); in some cases {Culeolus murrdyi), they probaljly form an accessory
. The Cambridge natural history. Zoology. 42 ASCIDIANS loop gives rise to the double or paired " vessels " seen branching through the test (see Fig. 17, v). The two vessels of a pair are â one blood-channel imperfectly divided by a connective-tissue septum. The l)lood courses out along one side, round the com- munication in a " terminal knob " at the end, and back down the other side. The " terminal knol)S " are very numerous, and form a marked feature in the outer layer of the test (Fig. 17, tJc); in some cases {Culeolus murrdyi), they probaljly form an accessory <jrgan of respiration, while in others (Botryllidae), they pulsate and aid in keeping up the circulation. The ectoderm is a simple epithelial layer (Fig. 16, ec\ It is turned in for a short distance at the branchial aperture (mouth),. ^â¢'"â¢â â¢r'.;ov Fig. 17.âSection through the surface layer of test oi Ascidia 77ie7Ut(hi, x 50. W. Bladder cells ; , test cell ; , tenuiiial knobs of vessels ; v, vessels of test. and atrial aperture (cloaca), as a short stomodaeum and procto- daeum respectively, lined in each case by a delicate prolongation of the test. Fig. 24, A, p. 52, shows the relations of ectoderm, mesoderm, and endoderm in a section through the antero-dorsal part of the body. The cavity marked ;r is a portion of the atrial cavity lined by ectoderm, and must not be confounded with a coelom. The absence of a true coelom in tlie mesoderm will be noticed in this and other figures, and yet the Tunicata are Coelomata, although it is very doubtful whether the enterocoel which has been described in the development of some is ever found. The coelom is in any case largely suppressed later, and is only re- presented in the adult by the pericardium and by small cavities in the renal and reproductive organs and ducts. Body-Wall and Cavities of the Body.âThe name " mantle " is given to the ectoderm with tlie parietal mesoderm which form
Size: 2659px × 940px
Photo credit: © Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
