OF THE HYDROMEDUSiE. 365 



layer of ectoderm, the radial string - of the adult. A sensory tentacle, with a single 

 otolith now grows out from the tip of each lohe, and the free edges of the lohes bend 

 down towards the mouth, thus forming a shallow circular sub-umbrella around the base 

 of the proboscis, and at this stage the larva detaches itself and escapes into the water 

 as a medusa, fig. 4, with an enormously long proboscis, a shallow sub-umbrella, four 

 long and four short tentacles, and alternating with the tentacles, eight marginal lobes, 

 each of which ends in an auditory tentacle, and contains a spacious diverticulum from 

 the central stomach. I have not been able to study the manner in which this cavity 

 becomes converted into the "festoon-canal," but this is probably formed by the growth 

 of an area of adhesion in the centre of each lohe, between the sub-umbral and the ex- 

 umbral endoderm. The pockets of the young medusae shown in PI. 43, fig. 4, must not 

 be compared with those of the adult Cunina which are of much later origin. After the 

 medusa is set free the umbrella grows very rapidly, while the proboscis remains without 

 change, so that the animal soon assumes the form shown in PI. 43, fig. 5, which is an 

 aboral view, copied from McCrady. The lateral pockets of the digestive cavity have 

 now disappeared and the central digestive cavity is nearly circular, but it soon 

 becomes folded in at its edge, between the bases of the tentacles, as shown in PI. 44, 

 fig. 2, in which figure the right half is an oral, and the left an aboral view. The eight 

 inter-tentacular notches on the free edge of the stomach now deepen rapidly as shown 

 in fig. 3 (copied from McCrady), and thus give rise to the eight stomach pockets of the 

 adult, while a thickening on the oral surface around the circumference of the stomach 

 marks the rudimentary reproductive organs, which soon spread over tVfo "wiiole oral 

 surface of the pockets. The long proboscis of the larva soon disappears, so that the 

 stomach becomes a fiat pouch with a contractile mouth in its centre, but in the adult 

 the oral wall of the stomach again becomes drawn downwards to form a pendent pro- 

 boscis. 



The life-history of Cunocantha octonaria may now be briefly summarized as follows: 

 The larva is a ciliated swimming organism, with a mouth, a long proboscis and two 

 opposite tentacles. It soon develops two more tentacles, loses its cilia, and becomes 

 a hydra with a greatly developed proboscis and with its aboral extremity reduced to 

 a small prominence, from which other hydras are budded. There is no sessile stage, but 

 the locomotor hydra makes its way into the bell of a Turritopsis, where it fastens 

 itself by its tentacles, and lives as a parasite. It then becomes directly converted into 

 a medusa by the outgrowth of an umbrella around its tentacular zone, and escaping into 

 the water begins its medusan life. Before it becomes a medusa it produces other 

 larvae by budding, and all these become medusa?.. The state of our knowledge of the 

 development of other Karcomcdusa", especially of Polyxenia and ^Eginopsis, indicates 

 that the parasitic habit of the larva? is not primitive but recently acquired, and the ten- 

 dency to multiply asexually has probably been also secondarily acquired by the larva as 

 an adaptation to its parasitic life. In the case of our Cunocantha all the larva? become 

 medusa', and there is therefore no true alternation of generations, but in the cast- of 

 the Cunina studied b} r Uljanin (GO), and afterwards by Metschnikoff, the adaptation 

 to a parasitic habit is much more perfect, and the larva which hatches from the egg and 



