MIDDLE CAMBRIAN. 39 



fission of Stomobrachium mirabile (probably a young form of Mesonema ccer- 

 uleseens). He states that the stomach divides first, and that simple animals 

 were seen in which this organ was folded and completely divided. Then a 

 furrow began to appear on the disk above the involution of the stomach, 

 that sank deeper and deeper until complete fission resulted. These animals 

 also divide at right angles to the first division. 1 



Kolliker's observations appear to have received little attention, and 

 they were not sustained by other discoveries until those of Davidoff and, 

 after him, those of Dr. Arnold Lang were made. Dr. Lang published a 

 memoir on Gastroblasta raffaeli in 1886. 2 He states that almost every indi- 

 vidual possessed more than one stomach, and a changing number of tentacles 

 and radial canals. Not a single example showed a true radiating arrange- 

 ment of organs. A large individual had nine stomachs and a large number 

 of tentacles and radiating canals; of the latter some were connected with 

 the stomachs and others were closed centripetal canals penetrating a longer 

 or shorter distance into the subumbrella, toward the middle of the body. 

 New stomachs were developed on some of the centripetal canals. Fission 

 begins by the doubling of the peripheral pustules (which inclose the otoliths) ; 

 then the canal uniting the two oldest stomachs is absorbed, and the canal 

 systems of the two stomachs are connected only through the exterior ring 

 canal. Then on the rim of the disk, at the double pustule, an invagina- 

 tion begins, which penetrates deeper and deeper until it meets another 

 similar but smaller incision which started from the opposite side. When 

 this is accomplished the medusa is divided into two individuals. 



The fission described by Kolliker and Davidoff occurred in true radi- 

 ating medusae; but G. raffaeli is an irregular form in the adult, resulting 

 from fission in a very young stage of the medusa, which was a true radiat- 

 ing form. This is Lang's interpretation, based on the results obtained by 

 Davidoff. To illustrate Lang's observations a number of figures have 

 been copied from his memoir and reproduced on Pis. XIX and XX. 



Prof. W. K. Brooks showed me a sketch of a specimen of Platypyxis 

 foliata McChesney, which he studied at Beaufort, North Carolina, in which 

 there were two stomachs and mouths, very much like those of Gastroblasta 

 raffaeli. He suggested that it and Dr. Lang's species might be identical. 



Professor Brooks has called my attention to the results of a study by 



1 Zeitschr. fur wiss. Zoologie, 1853, p. 235. 



- Jenaiscbe Zeitscbr., Vol. XIX, part 4, pp. 735-763. 



