290 DISCOVERY REPORTS 



sixteen eradial minute bulbs without ocelli ; but this complete number of appendages 

 is only seen in a few of the largest specimens examined, 8^-9 J mm. high. In the 

 smallest specimens observed, about i mm. high, only two of the perradial tentacles are 

 developed ; they are placed opposite each other ; the two other perradial bulbs are large 

 and have ocelli, and very soon they develop into tentacles which, when the medusa is 

 3-4 mm. high, have attained the same size as the two first ones. The four interradial 

 tentacles are indicated in the youngest stages as minute bulbs which soon grow larger 

 and become provided with ocelli; they are developed into small tentacles when the 

 medusa is 3-4 mm. high, but even in the adult the interradial tentacles are, as a rule, 

 distinctly shorter than the perradial ones ; only in some few of the largest specimens 

 the eight tentacles are of equal length. The eight adradial bulbs begin to appear in 

 specimens 3-4 mm. high ; they soon become provided with ocelli, but they are never 

 developed into tentacles, and even in the adult some of them may still be very small. 

 The development of the small eradial bulbs proceeds in a rather irregular way ; the 

 smallest specimen, in which a few eradial bulbs are observed, is 4.I mm. high, but on 

 the other hand there are some of larger size, about 6 mm., which have no traces of 

 eradial bulbs, and even in adult specimens the full number of sixteen is rarely present. 



Irregularities in the development of the tentacles and marginal bulbs are sometimes 

 observed. In some specimens one or more of the quadrants does not possess a single 

 interradial tentacle, but has two tentacles, dividing this quadrant of the umbrella margin 

 into three equal sections. In each of these sections a rudimentary bulb is developed, the 

 middle (interradial) one always remaining very small, whereas the two lateral ones are 

 larger and have ocelli. If this arrangement of the tentacles is carried through in all of 

 the four quadrants the medusa will have twelve tentacles, eight rudimentary bulbs with 

 ocelli, and four minute, interradial bulbs without ocelli. Two such individuals, in 

 which a "duodecimal" arrangement of the tentacles is carried through in all four 

 quadrants, are contained in the collection (both 7 mm. high) ; in other specimens the 

 duodecimal arrangement is only found in one, two, or three quadrants. The largest 

 specimen observed (10 mm. high) has ten tentacles, nine large bulbs with ocelli, and 

 twelve minute bulbs without ocelli. Similar irregularities in the arrangement of the 

 tentacles are observed in other species of Pandeidae (Kramp, 1926, pp. 77, 84, 93). 



Two other species of Halitholm are known, both belonging to the arctic seas: 

 H. pauper Hartlaub occurs on the north coast of Iceland, the west coast of Greenland, 

 and at Kamtchatka; H. cirratus is known from Greenland, Spitzbergen, and the 

 Barents Sea, and it also occurs as an arctic survivor in the cold basins of the Baltic. 

 It is very interesting that a species of the same genus has been found at the Falkland 

 Islands in the southern Atlantic. H. cirratus is considerably larger than H. iutermedius ; 

 it has a greater number of tentacles, and there are no ocelli on the basal bulbs. H. pauper 

 is only slightly larger than H. intermedins ; it has only eight tentacles : four large perradial 

 and four much smaller interradial ; ocelli are present, but are much smaller and less 

 conspicuous than in H. intermedins. Moreover, H. pauper is distinguished from 

 H. intermedins by the shape of the gonads, which have a conspicuous horseshoe-fold. 



