THE DEVELOPMENT OF THE EMBRYO INTO THE PARASITIC LARVA. 53 



projected beyond it (Fig. 24 />'), now becomes withdrawn or rather 

 invaginated towards the hinge of the shell, i.e., towards the dorsal 

 side (Fig. 25 A), and the whole body is thus divided into two halves, 

 each belonging to one of the valves of the shell (Fig. 25 A and B). 

 The mantle arises in this way and at this stage is remarkably large, 

 greatlj preponderating over the rest of the body, which only later 

 redevelops bv the outgrowth of the central portion of the body (Fig. 

 25). 



Before the withdrawal of the central portion of the body, four groups 

 of bristles were developed from the ectoderm on either side of the 

 embyro (Fig. 24 B, -so) ; during the invagination-processes just described 

 these organs lengthen and, in consequence of these changes, are then 



R 



B. 



mes 



->TO. 



Fig. 24. — Embryos of Atwdonta. (.1 founded on a figure by Flemming, somewhat 

 diagrammatic, /I alter Schikrhulz). .1, optic section with the outline of the shell 

 superimposed ; B, superficial view; eat, entoderm (archenteric rudiment); mes, 

 mesoderm ; s, unpaired shell ; sd, shell-gland ; so, sensory bristles ; w, ciliated area 

 [ventral plate]. 



found on the inner surface of the mantle. Each of these organs 

 consists of a long columnar cell which gives origin to a number of 

 long and fine sensory bristles (at first four to ten in number, later as 

 many as thirty) that perforate the thin ectodermal cuticle (Flemming). 

 These sensory organs are apparently of importance to the larva in the 

 process of attaching itself to the fish-host and are acquired at a late 

 embryonic stage. 



We may accordingly regard these sensory organs as differentiationE 

 of the mantle, and can hardly consider them to be related to the 

 velum, as Schierholz was led to believe on account of the position 

 of one of them. This particular organ occupies an isolated position 

 and has shifted in front of the oral aperture (Fig. 26 A). 



