The Journal of experimental zoology . When arranged incertain combinations and sequence, these activities form thereflexes with which the animal responds to stimulation. Fromsuch an aspect they will be considered in the description of thesensory reactions of the species. At present, however, my objectis to present the physiology of these movements in themselvesby examining the factors which are concerned in their production. 1. Siphon rivi closure Ascidia is usually described as possessing eight lobes on therim of the oral siphon and six on the atrial. These are shown infigure 1. All such phot
The Journal of experimental zoology . When arranged incertain combinations and sequence, these activities form thereflexes with which the animal responds to stimulation. Fromsuch an aspect they will be considered in the description of thesensory reactions of the species. At present, however, my objectis to present the physiology of these movements in themselvesby examining the factors which are concerned in their production. 1. Siphon rivi closure Ascidia is usually described as possessing eight lobes on therim of the oral siphon and six on the atrial. These are shown infigure 1. All such photographs and descriptions are of deadanimals and tell only a partial truth. In the normal, livinganimal under water, these lobes are not shrunken and collapsed, 244 SELIG HECHT but stand out expanded on the siphon in the form of thin lappets(fig. 5). Under special circumstances it is possible to secure a localcontraction of the region near an individual lappet. Ordinarily,however, the entire siphon rim shuts as a unit. This closure is. Fig. 5 Sketch of living, expanded Ascidia, to show the cheeks on the rightside and the protruding lappets on the siphon rims. conditioned by the presence of well-defined ridges and folds inthe test, along which the contraction takes place. An end-on viewof a nearly closed oral siphon (fig. 6) shows that the alternationof folds and ridges depends on a surprisingly accurate pattern,which involves thick and thin portions of the supporting test. PHYSIOLOGY OF ASCIDIA ATRA LESUEUR 245 The closing of the siphon rim, however, is more than a merepuckering together of its edge due to the action of circular musclefibers. The rim is not only pulled together, but is also drawndown toward the body of the siphon. This is due to the actionof longitudinal muscle fibers which, in the siphon, lie nearer thecavity of the siphon than do the circular muscles. The siphon rim in Ascidia is so opaque that it was impossibleactually to observe the action of the two sets of
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Keywords: ., bookauthorwi, bookcentury1900, bookdecade1910, booksubjectzoology
