91 



Free Medusa of 

 Laphcea cornuta, 

 magnified 15 di.a. 



a figiire of the adult Medusa, see Agassiz (L.) in Mem. American 



Academy, Vol. IV. Plate 7. The only species of the Laodicece Ag. 



which I have found young enough to show positively what the 



number of tentacles of the first set was, is the Medusa of Laphcea 



cornuta Lamx., fig. 4, in which the formula for the tentacles for 

 Fig. 4. « half the circumference is, Tj, ^g, T^, t^, T^ ; the pres- 

 ence of eight tentacles at that time is expressed by 

 the formula 2 t = 2 T^ -}- 2 Tg + 4 <3 = 8 <. This 

 species is closely allied to AtracUjlis rejjens Wright, 

 and I am inclined to believe that both may prove to be 

 the young oi Laodicea-like Medus£e. It will be very 

 interesting to see how this order of succession of the 

 sets of tentacles is modified in Laodicea calcarata 

 A. Ag., in which we have cirri, and club-shaped bodies 

 between the tentacles, as in Thaumantias mediter- 

 ranea Gegenb. Unfortunately I did not succeed in 

 finding any Laodicea calcarata young enough to 

 throw any Hght on this subject; from the youngest 

 specimen I met with, I am convinced that the first set 

 of tentacles does not consist of more than four, or 

 perhaps even only of two tentacles, specimens measur- 

 ing one quarter of an inch in diameter having not 



more than seven tentacles between every two chymiferous tubes. 

 In the Eucopidm Geg. we find a much greater difference in the 



number of tentacles of the first set. In Obelia commissuralis McCr., 



fig. 5, there are sixteen tentacles in the first set. In Eucope diaphana 

 Fig. 5. _4g. (see fig. 7), and in two other species closely 



allied to it, the first set consists of twenty-four 

 tentacles ; in another Hydroid belonging to this 

 family we find a young Medusa escaping from 

 the reproductive calycle with no less than forty- 

 eight tentacles, as in fig. 6, which represents a 

 quarter segment of a 



Obelia commissuralis new species of Eu- 

 cope f We know nothing of the adult 



condition of Obelia, fig. 5, and of the 



Medusa of fig. 6. But as we have no 



less than three species of Eucope on our 



coast, all escaping from the reproductive 



calycles with twenty-four tentacles, and 



as we now know several species of Obelia, 



all having sixteen tentacles in the first set, 



and still another, fig. 6, in which the first 



set has forty-eight tentacles, this difference ' Eucope ? 



in the number of tentacles of the first set would seem to be a;eneric. 



