654 KECOIID OF CURRENT RESEARCHES RELATING TO 



which prevails among the Traehomedusa? and the Narcomcdusae, they 

 possess as great a power of extension and contraction as may be found 

 in the tentacles of many Leptomeduste (Thanmantidas, &c.). These 

 four perradiate tentacles contract independently of the others, and 

 seem to form a different system. All the tentacles are armed along 

 their length with minute thread-cells, which are set in close, somewhat 

 spirally arranged, warts. 



The lithocysfs or marginal vesicles are, in adult specimens, about 

 128 in number. They are situated near the umbrellar margin of the 

 velum, between the bases of the tentacles, and are grouped somewhat 

 irregularly, so that their number has no close relation with that of 

 the tentacles. They consist of a highly refringent spherical body, 

 on which may be usually seen one or more small nucleus-like cor- 

 puscles, the whole surrounded by a delicate transparent and structure- 

 less capsule. This capsule is very remarkable, for instead of pre- 

 senting the usual spherical form, it is of an elongated pyriform shape. 

 In its larger end is lodged the spherical refringent body, and it thence 

 becomes attenuated, forming a long tubular tail-like extension which 

 is continued into the velum, in which it rims transversely towards its 

 free margin, and there, after usually becoming more or less convo- 

 luted, terminates in a blind extremity. 



The marginal nerve-ring can be traced running round the whole 

 margin of the umbrella, and in close relation with the otolitic cells. 

 Ocelli are not present. 



The generative sacs are borne on the radiating canals, into which 

 they open at a short distance beyond the exit of these from the base 

 of the manubrium. They are of an oval form, and from their point 

 of attachment to the radial canal hang down free into the cavity of 

 the umbrella. Some of the specimens examined contained nearly 

 mature ova, which, under compression, were forced from the sac 

 through the radial canal into the cavity of the stomach. 



While some of the characters described above point to an affinity 

 with both the Trachomedusfe and Narcomedusfe, this affinity ceases 

 to show itself in the very important morphological element afforded 

 by the marginal bodies. In both Trichomedusfe and Narcomedusse 

 the marginal bodies belong to the tentacular system ; they are meta- 

 morphosed tentacles, and their otolite cells are endoderraal, while in 

 the Lcptomedusfe, the only other order of craspedotal Medusae in 

 which marginal vesicles occur, these bodies are genetically derived 

 from the velum. Now in Limnocodium the marginal vesicles seem to 

 be as truly velar as in the Leptomedusae. They occur on the lower 

 or abumbral side of the velum, close to its insertion into the umbrella, 

 and the tubular extension of their capsule runs along this side to the 

 free margin of the velum, while the delicate epithelium of the abum- 

 bral side passes over them as in the Leptomedusa3. It is true that 

 this point cannot be regarded as settled until an opportunity of 

 tracing the development is afforded ; but in very young specimens 

 which Professor Allman examined he found nothing opposed to the 

 view that the marginal vesicles were derived, like those of the 

 Lcptomedusa", from the velum. 



