I9I5-] 



Fauna of the Chilka Lake : Coelenterates. 



99 



Fig. 8. — Acromitus rabanchatu, 

 sp. nov. 



A very young medusa with the 

 bell everted upwards. 



In our smallest specimens the disk is flat and membranous, with ouly a slight 

 convexity in the central region of the exumbrella. The margins can, however, be 

 everted upwards so that the structure becomes deeply 

 concave, resembling a chalice in form (fig. 8). The 

 muscular system is poorly developed and that of the 

 canals is still in a primitive condition. The sixteen 

 radiating canals are well developed, but they open 

 outwards directly into a circular canal on the peri- 

 phery. The walls of the latter canal are irregular in 

 outline and somewhat indefinite projections can already 

 be detected, representing the anastomosing channels 

 that will be developed later. The actual margin is so 

 delicate that it is invariably injured in specimens taken 

 in a tow-net, but the rhopalar lappets are relatively 

 large and conspicuous and the velar lappets short and 

 broad and perhaps not very clearly separated. There 

 appear to be four in an octant. The actual rhopalia 

 are well-developed, but the furrowed pit above them is represented only by a slight 

 depression in the exumbrella. 



The most interesting features of these young medusae are to be found in the 

 mouth-arms. The arm-disk has already assumed its final shape, but the ostia are 

 relatively smaller than in the adult and are not protected by depending processes of 

 the sub umbrella. These processes do not appear until a much later stage in post- 

 larval development is attained, and the ostia remain relatively small until the bell is 

 considerably larger. In the smallest specimens the arms themselves (pi. viii, fig. 2) 

 are still in the Semostoman stage and may be compared with those of the adult medusa 

 in Aurosa. They are united in a circle at their base to a slightly greater relative 

 extent than in the adult, to form what may be called a short manubrium, and are 

 arranged in four pairs. Each arm is an elongate, membranous, flattened process of the 

 margin of this manubrium, bilobed at the distal extremity and having the tips of the 

 lobes slightly everted. The lobes are rounded and do not diverge widely. The inner 

 (endodermal) surface is concave and a single row of minute capitate tentacles run 

 round the whole arm (including the lobes) , and also along the margin of the manu- 

 brium between the bases of the members of each pair of arms. The tentacles are 

 least numerous in the latter position. 



It would be out place to discuss the post-larval development in any great detail, 

 but one or two points of general interest may be noted. It may be stated firstly 

 that there appears to be very little correlation of a definite kind in the origin or 

 full elaboration of different organs in different individuals. In some very small 

 specimens the canal-system is already more elaborate than it is in others of much 

 larger size; the bell is much deeper, and has a shape more near that of the adult, 

 in some young examples than it has in others of more advanced development as 

 regards the canal-system; the terminal filaments of the arms rarely appear at the 



