OCR Text |
Show f)84 MR. W. BATESON ON ABNORMAL [June 17, apex of this horn is a small circular pit which is apparently closed by membrane. The other appendages are normal. A considerable number of cases similar to the above have been collected, especially by Kraatz, Mocquerys, and Jayne. JV,-Antedon rosacea with Abnormal Repetition of the Brachial Structures. This individual was found amongst a number of other specimens of A. rosacea collected by a party that had been dredging for the Plymouth Laboratory in the Hamoaze, near Beggar's Island. In normal individuals of this species the arms after leaving the radial plates do not again divide, but are continued to their ends as a single row of brachial plates, which bear pinnules on either side alternately. The present specimen, however, bears two arms, which, after being continued normally for a certain distance, break up into several secondary arms. The diagram (fig. 4, A ) shows the relation of these two varying arms (lettered b2 and <?,) to the mouth and anus. It is seen, therefore, that they are symmetrically placed. As I am inexperienced in the use of Crinoid terminology, I sent this specimen to Dr. P. H. Carpenter, who has very kindly supplied the following description of it:- " The abnormal arms of this remarkable specimen are symmetrically placed as regards the mouth and anus, being the posterior arms, b.2 and ex, of the two anterolateral rays. The arm b2 has been regenerated at the syzygy in its 15th brachial. But the pinnule on the new epizygal is on the same side (abradial or outer) as that on the 14fh brachial, and not opposite to it as would normally be the case, so that there are two pinnules in succession on the same side of the arm. The next twelve pinnules alternate regularly on opposite sides, those of the 19th and 27th brachials having much enlarged basal joints. That on the 28th brachial is considerably larger than its predecessors and more like a bifid armlet. It commences with five large joints, the last of which bears two pinnules, the one continuing the maip axis being rather stouter than its fellow. The 29th brachial is a syzygy and its epizygal axillary. The abradial or outer facet bears an arm, of which some 75 joints remain. It has pinnules on the 2nd and 4th, the latter oi which is a syzygy; but there is none on the 3rd, which would normally be a syzygy and bear a pinnule. On the larger, adradial, facet of the axillary 29th brachial is another axillary (30th br.), but without a syzygy. One of its facets bears the continuation of the primary arm, on the, next joint of which (31st br.) is an abnormal trifid armlet with three enlarged basal joints, the second being a syzygy with a pinnule- stump on the epizygal, while the fourth bears two pinnules. The 32nd brachial is again axillary with a syzygy, its epizygal bearing two subequal arms of some 60 joints each. The second joint of the left-hand one which continues the primary arm has a bifid pinnule with its basal joints enlarged, and the following pinnules alternate regularly on opposite sides. The arm borne on the |