586 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Pennaridse, Tubularidae, Clavidas, Bougainvillidse, and Campanularidse. 

 The comparative morphology of the colonies that form gonophores and 

 the comparative embryology of the gonophores lead to the conclusion 

 that the evolution of Athecata and Thecophora alike started from forms 

 which had already established an alternation of generations between 

 sedentary polyps and free-swimming sexual individuals. The latter had 

 the general structure and typical development of medusoids. Probably 

 also the ancestral forms showed tetramerous symmetry. 



The typical stages in medusoid-development are the following : (1) 

 The formation of the ectodermic " glockenkern " (sub-umbrellar primor- 

 dium) and the endodermic radial diverticula (primordium of the urn- 

 brellar endoderm) ; (2) the formation of the velar plate from the roof 

 of the bell-cavity, the manubrium, etc. ; and (3) the formation of the 

 marginal organs and tentacles, and the liberation of the velum. 



In diverse groups a reduction of the free-swimmiug medusoids ha& 

 come about, and the stages " eumedusoid," " cryptomedusoid," and 

 " heteromedusoid " may be distinguished, leading eventually to a simple 

 diploblastic sexual bud (styloid gonophores). Most of the gonophores 

 are demonstrably reduced medusoids. In a few cases it is doubtful 

 whether the gonads are produced from much reduced gonophore buds 

 (medusoid buds), or from modified polyp-buds, or are simply organs 

 arising de novo on polyps. 



New Genus of Unstalked Hydroids.* — A. H. Clark describes 

 Comatilia irido-metriformis g. et sp. n., a peculiar form from deep water 

 between the Bahamas and North Carolina. According to the structure 

 of its oral pinnules and brachials it belongs to the Coniasteridse. Judged 

 upon its other characters it might be placed in the Antedonidge, Hiniero- 

 metridfe, Thalassoinetridas, or Tropiometridae. Certain features find a 

 parallel only in Rhizocrinus and Bathycrinm. On the whole it is a. 

 Comasterid. 



Porifera. 



Reactions of Sponges t — G. H. Parker has made a series of interest- 

 ing experiments with Stylotella heliophila Wilson, a monaxonid demo- 

 sponge, and gives the following summary of his results. 



Under natural conditions Stylotella closes its oscula and contracts its 

 flesh when, at low tide, it is exposed to the air. Its outer surface is 

 perforated by many ostia which lead to large sub-dermal cavities ; these 

 in turn connect through incurrent canals with the flagellated chambers 

 from which excurrent canals pass to the gastral cavity and the osculum. 



The flesh of Stylotella contains many myocytes, which are arranged 

 as sphincters around the ostia, internal cavities, and osculum. These 

 sphincters apparently work against the general elasticity of the flesh and 

 not against radiating systems of myocytes. The oscula close in quiet 

 sea-water, on exposure to air, on injury to neighbouring parts, in solu- 

 tions of ether (O'o p.c), chloroform (0*5 p.c), strychnine (ig^oo)- 



* Proc. U.S. Nat. Museum, xxxvi. (1909; pp. 361-73 (l|pl.). 

 t Joum. Exper. Zool., viii. (1910) pp. 1-41 (3 figs.). 



