ioo Memoirs of the Indian Museum. [Voi,. V, 



same time on all the arms of the same medusa, and are frequently absent in indivi- 

 duals of later growth than in some of those in which they are fully formed ; the 

 lateral sensory filaments of the arms usually appear later than the terminal ones, but 

 can sometimes be detected in the form of minute buds before the latter make their 

 appearance. 



A second point of interest lies in the fact that in the development of the canal- 

 system the inward projections of the peripheral plexus connected with the adradial 

 canals first make their appearance as irregular processes of the primitive circular 

 canal and are in no way connected with the radiating channels. Each projection 

 is formed in two halves, one half on each side of the canal with which the whole is 

 ultimately to fuse. Even after the two halves have become joined to the two sides 

 of the radiating canal, it runs straight through them and maintains its identity to 

 the margin of the disk. This condition prevails for a considerable period and may 

 occasionally be found persisting as an abnormality in one or more octant of a large 

 medusa. 



But the most interesting feature of our series of young specimens of Acromitus 

 lies in the clear manner in which it illustrates the evolution of the Rhizostomatous 

 mouth-arm (plate viii, figs. 2 to 3Ô). The peculiarities of the structure of this organ 

 are due in the first instance to unequal growth in its different parts. The everted 

 terminal lobes of the arm of the young medusa grow more rapidly than the simple 

 basal part, and the margins in both regions grow more rapidly than the middle 

 portion. The first consequence of the accelerated growth of the terminal lobes is 

 that the whole arm is definitely folded inwards along the middle line, while the 

 fact that the margins become longer than the middle region causes them to be 

 thrown into a series of short transverse pleats. This double folding causes certain 

 parts of one side of the arm to be brought into close contact with the corresponding 

 parts of the other side, and also certain parts of each margin to be pressed against 

 others on the same side ; but prevents the whole of one vertical half coming into con- 

 tact with the whole of the other. In fact, a central vertical canal is left open 

 down the mid-ventral line of the primitive arm, while lateral canals of smaller 

 calibre diverge from it obliquely to the margin on either side. The whole figure 

 thus formed is pinnate. Simultaneously with the production of this system of 

 canals a great increase in the bulk of the mesogloea . of the arm takes place. 

 Where endoderm meets endoderm in the folding, the two surfaces fuse together and 

 are invaded by mesogloea, which cuts off one canal from another, leaving those 

 endodermal tracts free that have not been in contact. The endoderm in the interior 

 of the greatly strengthened and thickened arm that is thus produced is now confined 

 to the lining of the vertical and lateral channels formed by the folding of the 

 originally membranous structure and its consolidation in the manner indicated. The 

 distal extremities of the lateral canals remain open and form two linear series of 

 mouths, extending, one on each side of the new margin, down the arm and along 

 each of the terminal lobes. 



Yet another folding takes place owing to the growth of these lobes. At first 



