Mr. Macdonald on the Anatomy of Macgilllvraya. 359 



Attention is then directed to what has been called (but by a mis- 

 apprehension) the •' stirrup-shaped" armature of the genera Rotifer^ 

 Philodina, Actinurus, &c. Here, however, the organs are proved to 

 have no essential diversity from the common type ; their analogy 

 with those last described being abundantly manifest, though they are 

 still further disguised by the obsolescence of the manubria. 



Floscularia and Stephanoceros, the most elegant, but the most 

 aberrant forms of Rotifera, close the series. The mastax, in these 

 genera, is wanting; and in the former genus the incus and the 

 manubria are reduced to extreme evanescence, though the two-fin- 

 gered unci show, in their structure, relative position and action, the 

 true analogy of these organs. 



Having thus shown that there is but one model of structure, how- 

 ever modified or disguised, in the manducatory organs of the Roti- 

 fera, the author proceeds to the question of their homology. He 

 argues on several grounds that they have no true affinity with the 

 gastric teeth of the Crustacea, though he states his conviction 

 that the Rotifera belong to the great Arthropodous division of 

 animals. 



, Jt is with the Insecta that the author seeks to ally these minute 

 creatures ; and, by a course of argument founded on the peculiarities 

 of structure already detailed, he maintains the following identifica- 

 tions : — that the mastax is a true mouth ; that the mallei are mandi- 

 bles ; the manubria possibly representing the cheeks, into which they 

 are articulated ; that the ramioi the incus are maxillae ; and that the 

 fulcrum represents the cardines soldered together. 



While the author maintains the connexion of Rotifera with 

 Insecta, through these organs in their highest development, he 

 suggests their affinity with Poiyzoa, by the same organs at the 

 opposite extremity of the scale, since the oval muscular bulbs in 

 Bowerbankia, which approach and recede in their action on food, 

 seem to represent the quadriglobular masses of Limnias and Rotifer^ 

 further degenerated. 



If this affinity be correctly indicated, the interesting fact is appa- 

 rent, that the Poiyzoa present the point where the two great parallel 

 divisions, MoUusca and Articulata, unite in their course towards 

 the true Polypi. > 



March 22. — The Lord Wrottlesley, President, in the Chair. "^• 



" Further observations on the Anatomy of Macgillivrayat Chele- 

 tropis, and allied genera of pelagic Gasteropoda." By John Denis 

 Macdonald, Esq., R.N., Assistant-Surgeon H.M.S.V. ' Torch.' 



The author states, that in a late voyage from Sydney to Moreton 

 Bay, specimens oi Macgillivraya, Cheletropis, and a few other genera 

 of minute pelagic Gasteropoda, apparently undescribed, were daily 

 taken in the towing-net, and afforded him an opportunity of more 

 precisely determining the mode of attachment of the ciliated arms 

 which he had at first presumed to be naked branchiae. 



In his former paper "^ it was stated, more particularly of Cheletropis 

 * Annals, p. 232. 



