INTERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 771 



ment of the formation of the oesophagus, there are only two enclodermal 

 cells. As the embryo changes from its ovoid form, we get a flattened 

 pre-oral, a conical post-oral, and a rounded posterior region. With 

 high powers it is possible to see, at some distance from the mouth, a 

 double circlet of delicate cilia, supported on two special rows of ecto- 

 dermal cells. 



The two large ectodermal cells divide, and form the posterior 

 endodermal mass ; a double pre-oral ciliary cii'clet becomes developed. 

 Soon the whole siu-face of the embryo is covered with cilia, the only 

 naked region being a portion of the hinder part of the dorsal surface. 

 The ectodermal cells begin to form a shell-gland, and this, at a later 

 stage, forms a deep thick-walled saccule with a narrow cylindrical 

 lumen ; its orifice and margin are covered by a delicate chitinous 

 cuticle, which represents the earliest rudiment of the shell, and indi- 

 cates thereby the primitively unpaired condition of this organ. 



From the primary mesodermal cells two or three smaller ones 

 have been budded off on either side, and pushed forwards ; tho 

 characters of these parts strongly call to mind the arrangements which 

 obtain in Criodrilus. The shell becomes double while still very thin, 

 and almost cuticular in character. 



At a somewhat later stage the form of the body and the rudiments 

 of the organs call to mind the disposition of parts in the trochophore 

 stage of the Annelid-larva ; the stages next succeeding arc very 

 markedly affected by the development of the shell, which has grown 

 considerably, and about this time the double pre-oral circlet of cilia 

 disappears. The development of the musculature is now rapidly 

 going on ; and a number of separate parts soon become well marked ; 

 still do the primitive mesodermal cells retain their large size. As tho 

 shell grows, takes on a yellowish coloration, and becomes marked by 

 parallel lines of growth, the characters of the ciliation become much 

 changed ; cilia have disappeared from the frontal area and from tho 

 ventral surface ; in the oral region, pre-oral, post-oral, and adoral 

 zones are to be distinguished. 



As the larva at this stage is completely trochophoral, save only as 

 regards the presence of a shell and a mantle to indicate its molluscan 

 ancestry, we have to look for a similarly well-marked excretory organ ; 

 this, just like the kidney of the Trochophore, is to bo found at tho 

 anterior end of the mesentery, where it forms a longish organ, with a 

 delicate lumen, and ciliated internally. As this body elongates it 

 becomes connected with the ectoderm and gets to open to the exterior 

 by means of an orifice in this layer. 



As we cannot follow the author through all his further details, wo 

 will pass to the concluding part of this descriptive chapter, in which 

 he speaks of tl;e developnient of the gills. In the maiith; cavity, at 

 tho sides of the trunk, there appears a ridge of ectoderm, wliich belongs 

 to the inner lamella of the mantle-fold. Later on, the hinder portion 

 of this branchial ridge gets set at right angles to the anterior, and at 

 the angle the rudiment of tho gill is best developed. At a point near 

 the free cd^o the two layers, of wliich tho fold is composed, bccomo 

 thinner ; depressions ap})car in this which lead to tho breaking up of 



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