28 



MERTENSIA OVUM. 



fonned, the digestive cavity (d) and the lateral tubes (Z) are well de- 

 fined. (Fig. 32.) In a view from below of this same individual (Fig. 33) 

 we notice the narrowing of the large ambulacral pouch into somewhat 



more circumscribed 

 tubes. In the next 

 stage which is here 

 represented (Fig.34), 

 the ambulacra have 

 assumed still more 

 the aspect of tubes, 

 the funnel has elon- 

 gated, the tentacles 

 have begun to send 

 out lateral processes, 

 the lateral tubes ex- 

 tend nearly to the 

 level of the mouth, 

 the actinal part of the young Medusa having 

 taken a still greater development, and having 

 become as long as the ambulacral part. The 

 tubes, both ambulacral and lateral, when seen 

 from below (Fig. 35), are also more narrowed 

 and better circumscribed. In the next stage the development of the acti- 

 nal part of the spherosome (Figs. 36, 37) has become so striking, that we 

 Fig. 35. cannot fail to recognize in the young Acaleph a 



Mertensia. The difference between the coehac and 

 diacoeliac axis is quite prominent, giving to the 

 animal, when viewed from the broad (Fig. 36) or 

 narrow side (Fig. 37), a totally different aspect. 

 The tentacular apparatus differs from that of Pleu- 

 robrachia m being hmited to the abactinal part of 

 the spherosome, and not extending towards the 

 actinostome, as in Pleurobrachia. In the young 

 stages the lateral tubes are still quite close to the 

 digestive cavity, and do not yet flare out, as in the 

 adult. (See Figs. 29, 36.) The ambulacra are very nearly equally 

 developed, the tentacular pairs and the median tubes differing but 

 slightly in length. The tentacles are lashed and covered with large 

 orange pigment-spots, similar to those of the rows of locomotive 



Fig. 32. Somewhat more advanced Mertensia, seen from the narrow side ; the lateral tubes, I, 

 are present. 



Fig. 33. The same as Fig. 32, seen from the actinal pole. 



Fig. 34. Still more advanced Mertensia, seen from the narrow side. 



Fig. 35. The same as Fig. 34, seen from the actinal pole ; the tubes ai-e circumscribed, and 

 the tentacular apparatus isolated. 



