97 



■with T'j ; the same applies to Tg. Soon after the tentacles of the 

 third set make their appearance, and in fig. 21 the formula is: 



h, T^, T2, «3, Ti, Tg, or, 2t=:4Tl4-4T2 + 4i3 = 12^• 



a branch ^c is then set off in the opposite ^'^^'- ^'• 



direction to ^c, which soon reaches the circular 

 tube at the point where the third set of tenta- 

 cles has already been formed. The last for- 

 mula becoming for this stage : 



/ * . / * » 



or, S t = 4 Tj -f 4 T2 + 4 Tg 1= 12 1 



We have thus in Willia a very peculiar order 



of development in the first three sets, modified 



by the manner of branching of the chymiferous Willia, more advanced 



tubes. than fig. 20. 



In other genera of the Tubularians, as Turritopsis McCr., and 

 Turris Less,* we find again the same regularity as in Staurophora and 

 Eucope; the sets of tentacles making their appearance in the same 

 order. In fig. 22, we have a young Turritopsis nutricula McCr., 

 Fig. 22. having only four tentacles. For a figure 



of the adult, see McCrady, Proc. Elliot 

 Soc, Plates 4 and 5. Fig. 23 is the 

 same species with sixteen tentacles. 

 The formula of fig. 23 is : 



Young Turritopsis. 



the tentacles are found arranged in 

 clusters, we have an entirely dif- 

 ferent mode of development. In 

 Bougainvillia superciliaris Ag. (for 

 a figure of the adult see Agassiz 

 (L.) in Mem. Am. Acad., Vol. IV. 

 Plate 1) the young Medusa, at the 

 time when it separates from the 

 Hydra, has two tentacles in the pro- 

 longation of each chymiferous tube, 

 as in fig. 24 ; the next sets are 



Tj, ig <2) ^3) Tj. In the Nemopsidce 

 Ag. and Bougainvillidce Lutk., in which 



Flo. 23. 



Young Turritopsis nutricula. 



* The Turris which occurs at Nahant is probably the Medusa digitalis Tab. 

 As the name Ttirris digitalis is pre-oooupied for an English species, I would pro- 

 pose the name of Turris vesicaria for the species found on our coast. 



PROCEEfilNGS B. S. N. H. — VOL. IX. 7 SEPTEMBER, 1862. 



