MUSEUM OF COMPARATIVE ZOOLOGY. 297 



Phialium. duodecimale, Haeckel. 



Figs. 17-31. 



The youngest larval stage of the medusa of P. duodecimale * found by us 

 has two long tentacles, which are situated opposite each other on the bell rim. 

 Each tentacle is accompanied by two tentacular spurs or filaments, which arise 

 from the bell margin near the tentacular bulb. The rudiments of two inter- 

 mediate tentacles are visible as simple projections on the bell margin. A 

 description of this larva, which resembles closely the planoblast, Loventlla, is 

 given below. 



The bell is tall, almost spherical, with thin walls. Its outer surface is 

 smooth. At the apex there is a remnant of the tube by which the medusa was 

 attached in an earlier stage to its hydroid. It has four simple narrow chymif- 

 erous tubes, which are destitute of sexual organs, f Proboscis short, small, with 

 closed mouth. There are four otocysts, each containing a single otolith, and 

 alternating on the bell margin with the radial tubes. 



The two larger tentacles are long, coiled in the distal two-thirds of their 

 length, and accompanied by flexible "spurs" or filaments, % which arise near 

 their bases. 



A still older medusa of P. duodecimale differs from the former in having 

 four well-developed tentacles, each of which is accompanied by a pair of lateral 

 filaments. The bell of this larva has a more pointed apex, which has thicker 

 walls than that of the preceding. Sexual glands, four in number, have made 

 an appearance upon the radial tubes near their junction with the circular ves- 

 sel. There are four otocysts, each alternating with a tentacle. The height of 

 the bell, when expanded, is about two thirds its diameter. When the bell walls 

 are contracted the height and diameter are about equal. 



In a medusa more advanced in growth than the last two, additional otocysts 



* Mr. Agassiz's suggestion (op. cit.), that his Euchrilota ditodecimalia belongs to a 

 different genus from McCrady's E. ventriculcuris, is supported not only by the differ- 

 ence in number of otocysts on the bell margin of the adult, but also by the character 

 of the development of the two. "Without adding any new facts to our knowledge 

 of these two forms, Haeckel has already suggested the name Phialium (orS.duth 

 \. Ag. The difference in the form of figs. 106, 107* (North American 



Acalephs), upon which Haeckel relies, together with the inflated or shrunken condi- 

 tion of the ovaries, for his two species of Phialium, docs not seem to me to warrant 

 the separation. I therefore retain the specific name to for both. 



t The sexual organs are well developed in Prof. Clarke's figure of the medusa of 



L. gracilis, Clarke, They are wanting in Hincks's drawing >•( L.dausa. (Clarke, 

 Mem. Bost. Soc. Nat. Hist., Vol. III. No. IV. Hincka, Ann. Hag. Nat. Hist., 

 VIII., 1871, p. 70, PL V. figs. -J--2 b .) 



t 1 have not followed a medusa of this age in its development into the following 

 stage. Both stages are common at the same time, and they appear to be the same. 

 Absolute proof is as yet wantiug. 



