48 ART. 4. N. YATSÜ : 



does not directly turn into muscular tissue, but proliferates cells 

 between itself and the outer layer. In the cells thus formed the 

 muscle fibres (shell muscles) are produced. The peritoneal layer, 

 on the other hand, remains throughout life as the sheath of the 

 muscles. van Bemmelen ('91) thought, however, that the 

 peritoneum itself changes into the muscular tissue. Between the 

 otocysts the peritoneum produces a thickening which according 

 to Brooks represents a nerve string connecting the otocysts, but 

 this conclusion cannot, I believe, be accepted. The thickening is 

 clearly composed of enlarged epithelial cells filled with highly 

 refractive granules. There is no evidence determining the true 

 nature of this thickened' ridge. I am, however, led to believe 

 that it is an area whose function is excretion. 



The supporting substance which as a rule is found between 

 the epidermis and the peritoneum must certainly be present in 

 the larvœ now under consideration, but it is still too thin to be 

 readily determined. As to the formation of the supporting sub- 

 stance, though there is no positive proof in Lingula, I believe 

 that it is secreted not only by either or both of these two layers, 

 but also by the cells which are proliferated by either of the 

 layers and are found in the adult imbedded in the supporting 

 substance. 



/. Blood Corpuscles. 



A few blood corpuscles were first noted floating about in the 

 body cavity at the end of the 7 p. c. stage. The corpuscles 

 rapidly increase in number and at the 8-9 p. c. stage several 

 hundreds of them are seen (PI. VI., Fig. 84.). At this time 

 only the ordinary kind of corpuscles is found, the leucocytes and 



