Geological magazine . zooecia continuous past the central (initial) zooecium. x 8.,, 6. Ditto. The initial zooecium and surrounding zooecium from the specimen shown in Fig. 5. x 12.,, 7. Ditto. Ditto, from another specimen, x 12.,, 8. Ditto. Ditto. ,, 9. Ditto. Underside of another specimen. x 6.,, 10. Ditto. Underside of and cross-section through another specimen, in which the individual zooecia are still traceable. x 6. IV.—On the Evolution of the Apical System in the Holectypoida. By Heebekt L. Hawkins, , , Lecturer in Geology, UniversityCollege, Eeading. (PLATE II.) A. Introducti
Geological magazine . zooecia continuous past the central (initial) zooecium. x 8.,, 6. Ditto. The initial zooecium and surrounding zooecium from the specimen shown in Fig. 5. x 12.,, 7. Ditto. Ditto, from another specimen, x 12.,, 8. Ditto. Ditto. ,, 9. Ditto. Underside of another specimen. x 6.,, 10. Ditto. Underside of and cross-section through another specimen, in which the individual zooecia are still traceable. x 6. IV.—On the Evolution of the Apical System in the Holectypoida. By Heebekt L. Hawkins, , , Lecturer in Geology, UniversityCollege, Eeading. (PLATE II.) A. Introduction, IN^ two previous papers in this Magazine (Hawkins, 1910 and 1911)I have dealt with some features in the comparative anatomy ofthe ambulacral and interambnlacral areas of the Holectypoida. Theonly other structure of the test for a comparative study of whichsufficient material is available is the apical system. The presentpaper deals with this structure. I intend to summarize the results Geol. Mag. 1912. Plaxk Jiemrosc, CoUo. Chalk Polyzoa. H. L. Haiokins—Apical System in the Holectypoida. 9 of my researches in a scheme for the classification and evolution ofthe group in a paper already partly prepared. B. The points to be examined. Tlie apical cycles of plates are of fundamental importance in anEchinoid. IS^ot only are they directly associated with the reproductivesystem, the water-vascular system, and some of the more delicate ofthe sensory organs of the animal, but it is from their outer marginsthat all the new coronal plates arise. The differences of structureAvhich are shown in the apical system in various groups must thereforebe indications of essential differences in the structure and developmentof both the skeletal and softer tissues of the body. From the fore-going sentences it will be realized that I differ considerably from theopinions expressed by Duncan (1889), and tend to regard the apicalsystem as a phylogenetic index of the first importance. Neverthele
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Keywords: ., bookcentury1800, bookdecade1860, booksubjectgeology, bookyear1864
