576 SUMMARY OF CURRENT RESEARCHES RELATING TO 



process of the modification of tlie cells into horny matter having 

 been clearly recognized ; " in the stem no outer epithelium can be 

 expected ; its absence is a proof that the chitinous sheath is a product 

 of the differentiation of the ectoderm." 



The chitinous theca therefore of Clytia is not homologous with 

 that of Cordylophora. Beneath this " epidermis layer " of Clytia 

 follows a deeper ectoderm layer, corresponding to the neuro-muscular 

 layer oiHydra, &c., which differs in the " body " and stem of the polyp ; 

 in the former there is a thick homogeneous layer to which the term 

 middle zone (" mittelzone ") is applied ; this in the stem and the disk 

 of the polyp becomes a distinctly cellular layer, one passing into the 

 other without any break ; the outer cellular layer of the stem is not 

 therefore, as might appear from a casual inspection of Klaatch's figure, 

 the equivalent of the outer ectoderm layer — the " epidermis-schicht," 

 but really represents the " mittelzone," and has nothing to do with 

 the formation of the outer chitinous layer which is formed by a meta- 

 morphosis of the " epidermis-schicht." 



The different conditions of the middle zone may be perhaps 

 explained by its different functions in the body and stem of the polyp 

 respectively ; in the body it undergoes alterations of diameter in 

 various stages of contraction, and from this fact appears to be rather 

 muscular than nervous in nature ; in the ^tem, on the other hand, the 

 rigidity of the chitinous investment would seem to render the 

 presence of a muscular layer unnecessary, and it is very possible that 

 the cells which in this region of the polyp represent the " muscular " 

 layer of the body are modified to form nervous structures which 

 receive impressions and control the movements of the muscular layer 

 of the body, with which, as has been already stated, they are in direct 

 connection. 



Structure of the Velellidse.* — M. Bedot finds that in young 

 Velellidae the two layers of which the crest of the pneumatocyst is 

 formed are not united together; they first appear as a fold of the 

 upper part of the pneumatocyst. The " liver " is of some complexity ; 

 on its upper or convex part there is a single layer of cells which is in 

 direct contact with the pneumatocyst ; below this is a lamella, in 

 which no cellular structure could be made out ; against this there are 

 applied the canals of the " liver," which form a kind of roof for a 

 large mass of cnidoblasts; the presence of these last demonstrates 

 that the so-called liver does not perform hepatic functions. Below it 

 there are again some canals which differ from the more superior by 

 being un pigmented; they are attached to a similarly structureless 

 lamella. The two sets of canals are connected with one another 

 through the substance of the organ. 



The complicated vascular system arises simply as two straight 

 canals which open into the marginal one ; they bifurcate at a short 

 distance from their point of insertion. In the course of their develop- 

 ment they become sinuous and give rise to a number of ramifyiug 

 cseca, which anastomose with those of the adjacent canals. 



* Arch, Sol, Phys. ct Nut,, xi, (1884) pp. 328-BO. 



