REVIEW. 



629 



best known papers I have never seen this description alluded 

 to, and the entire paper has curiously escaped Dendy^s notice. 

 Metschnikoff's pi. xxi, fig. 1, gives evidence of the flask- 

 shaped epithelium in the young Halisarca, corroborating 

 Merejkovsky's description, with which, indeed, the figures 

 of Schulze and similar ones of Metschnikoff's are at variance 

 only in interpretation. I have found this glandular ectoderm 

 in a horny sponge (Aplysina ?), an Axinellid, and a Renierid. 

 Von Lendenfeld's "mesodermal gland-cells" are certainly 

 nothing else in Calcarea, and as the descriptions are identical 

 they are probably the same in horny sponges ; many " sestho- 

 cytes " and similar structures in all likelihood have the same 

 foundation in fact. I therefore personally believe that the 

 typical ectoderm, not only of Calcarea, but of sponges, is a 

 glandular epithelium of flask-shaped cells with dilated mouths, 

 and that on external surfaces this is probably the usual form.^ 

 Mr. Denby has shared with most others the disadvantage of 

 working on specimens more or less shrunk by preservation in 

 alcohol ; to this shrinking, to generalisation from the epithe- 

 Hum of canals (more easy to observe than the defended 

 exterior), and to deduction of the existence of a flat epithelium 

 from mere hexagonal silver lines, I attribute the overlooking 

 of this structure by all but the two Russian authors. 



Dendy has not found cilia on the ectoderm of Homocoela, 

 and throws much doubt on the figures of Lendenfeld, where 

 they are invariably introduced. After long comparison of 

 Dr. von Lendenfeld's descriptions of Calcarea with the original 

 structures I have no hesitation in saying that his " low flat 

 plates filled only partially with protoplasm; from this plate 

 threads extend which pervade the cell-cavity," are com- 

 pletely imaginary. His flagella certainly do not exist in the 

 Calcarea I have examined ; they are probably a generalisation 

 from some structures he has seen in horny sponges. In 

 Aplysina (?) I have found threads standing vertically from the 

 ectoderm and precisely simulating flagella; they are undis- 

 1 I think that it may prove the primitive Metazoon ectoderm, and will 

 probably be found in various larvse. 



