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THE RELATION OF THE MADREPORITE TO THE PHYSIOLOGICAL ANTERIOR END IN THE TWENTY-RAYED STARFISH, PYCNOPODIA HELIANTHOIDES (STIMPSON)

1922; Marine Biological Laboratory (MBL); Volume: 42; Issue: 4 Linguagem: Inglês

10.2307/1536522

1939-8697

H. P. KJERSCHOW-AGERSBORG,

Cephalopods and Marine Biology

1. The physiological anterior end in Pycnopodia helianthoides corresponds to the posterior end of Ludwig's ground plan for Asterias. The anterior end possesses a relatively larger number of old (large) rays in early life than it does in late life. The new rays are added at two interpolation zones posterior to rays II., I.2. The madreporite is, as a rule, on the left anterior side; it varies in position, but it is always found on the anterior part of the dorsal disk, i.e., at the base of rays III., IV., or V. The socalled five primary rays (Ritter and Crocker), which, according to Ludwig, correspond to 3-IV., 2-III., 1-II., 5-I., 4-V., respectively, are not the posterior rays in adult life of Pycnopodia, but the anterior rays. Ray IV. is the anterior ray in adult life.3. The establishment of the physiological anterior end in Pycnopodia shows that the madreporic plate is anterior to the mid-dorsal and the anus posterior; this arrangement agrees fairly well with Delage and Hérouard's plan for the five-rayed starfish. It also corresponds with Cuénot's plan for the common starfish, with Jennings's "more or less permanent `set,' " and with Cole's "physiological anterior"; it is the reverse of Ludwig's asteroid ground-plan relative to the numbering of the rays.