260 



COMPARATIVE ANATOMY 



CHAP. 



has given rise to two or three smaller cells. The thin cuticular shell becomes 

 bivalvular by the appearance of a mediodorsal boundary line. 



A farther stage is distinguishe<l first by the appeai'ance of a small posterior 

 ectodermal invagination, the proctodseum, which produces the rectum and anus. 

 An ectodermal thickening, the neural plate, appears in the pretrochal area, carrying 

 three flagella. Some of the mesoderm cells become muscle cells (Fig. 216 G). 



The next stage may be called that of the Trochophora larva. This larva differs 

 from a typical Annelidan Trochophora only by possessing a shell, which now covers 

 the greater part of the bodj^, and bj' a mantle which appears, at first, posteriorly, and 



Pig. 217.— Older Larva of Teredo, from the riglit side (after Hatschek). Lettering as in Fig. 

 216. Ill addition, 14, rudiment of tlie digestive gland (liver) ; 15, preoral ciliated band (velum) ; 

 16, postoral ciliated band ; 17, primitive kidney ; 18, auditory vesicle ; 19, rudiment of the pedal 

 ganglion ; 20, rudiment of the gill ; 21, mesodermal streak. 



then at the sides, as a fold, and continues to grow from behind forward. The region of 

 the body which lies behind the cephalic area has sjiread out on each side to form a 

 broad fold, which becomes outwardly applied to the shell. The neural plate has 

 become multilaminar, and the ])roctodffium has broken through into the mid-gut. 

 The primitive mesoderm cells have given rise to a short mesoderm streak on each 

 side. At the anterior end of each mesoderm streak a somewhat long bod}', the 

 primitive kidney, has formed ; this contains a channel which opens externally, and 

 whose lumen is ciliated at a later stage. The rudiment of the digestive gland 

 appears in the mid-gut as a paired semi-spherical outgrowth. The body is no longer 

 ciliated all over ; cilia are retained only on the neural plate and in the anal region. 

 The double preoral ciliated band now becomes very distinct, and a postoral band is 



