ON MESENTERIES IN URTICINA CRASSICORNIS, 453 



31. The Development of the Mesenteries in the Actinian 

 Urticina crassicornis ^' . By James F. CIemmill, M.A,, 

 M.D., D.Sc. 



[Received August 18, 1919 : Read Novembei' 18, 1919.] 



(Text-figures 1-5.) 



The adult Urticma is remarkable for having its mesenteries 

 and tentacles apparent!}' arranged in lO-cj'cled symmetry (Dixon 4, 

 Faurot 5, Haddon 7), and on that account has been placed by 

 various authors among the Paractinepe. It is, however, a Hexac- 

 tinian, the arrangeiuent of whose mesenteries has become modified 

 during early growth. 



The species investigated was the large fleshy one with few 

 warts, occurring beyond tide-mark down to a depth of at least 35 

 fathoms, variously and often brightly coloured, attaining a size, 

 when fully grown, of over six inches in expanded disc diameter, 

 and having 160 tentacles of which 80 occur in the outermost 

 circle. The other circles from within outwards consist respec- 

 tively of 10, 10, 20, and 40 tentacles. Gosse (6. p. 211) thinks 

 that this is just the shore species (his Tealia crassicornis) modified 

 for living under water, though he also describes an apparently 

 identical foi'm as a separate species under the name Bolocera 

 eqites (6. p. 351). 1 find that in the Firth of Clyde both the 

 shore and the submerged forms shed their eggs prior to fertiliza- 

 tion (a distinction from Rhodactinia crassicornis (3. pp. 39, 41)), 

 that development is the same in both, and that cross fertilization 

 can occur between them. On general grounds I would have 

 judged that the shore and the submerged forms were varieties of 

 the same species had my account (see below) of the development 

 of the mesenteries in the latter agreed with that of Faurot 

 (5. p. 172) for the former. It appears to have been the submerged 

 species whose development was investigated by Appellof (1). 

 T. A. Stephenson, reviewing the nomenclature in a note to me, 

 concludes that the shore form should be called Urticina (Ehren- 

 berg) corlacea (Ouviei'), and tbe submerged form Urticina 

 (Ehrenberg) crassicornis (0. F. Miiiler), and that the two are 

 probably distinct species. 



My material consisted of (a) young specimens reared to the 

 12 mesenteried stage from eggs shed in the tanks at the Millport 

 Biological Station, and (b) growth stages dredged from c. 20 fms. 

 near the Station, the youngest of which had only 12 tentacles. 

 All were studied by the method of serial sections. Of the 8 

 Edioardsia mesenteries, the ventro-laterals are the earliest to 



* I have to express indebtedness to the Carnegie Trust for a grant towards 

 expenses of research wort, part results of which are given in this and the following 

 paper. 



31* 



