310 PROCEEDINGS OF THE ACADEMY OP [May, 



While McMurrich declares the iutima to be impermeable, both 

 Conklin aud Schonichen speak for the existence of pores through 

 which food may pass. These are best seen in the fresh intestine. 

 When the iutima is found with the posterior half of the test, one 

 has only to nioimt it in water to demonstrate clearly the pores iu 

 all parts. They are a little more numerous per unit area of sur- 

 face in the median than in the anterior portion. Owing to the 

 relatively uniform structure of the chitin and its high refractive 

 index, it is difficult to make out more than little pits at the sur- 

 face, i. e., the luminal end of the pores. In sections made from a 

 fresh intestine with a freezing microtome and mounted in gum- 

 arabic I chanced to get an oblique view of the pores, which supplied 

 the direct evidence that they actually perforate the intima. One 

 can in this way measure both their length and diameter. The 

 former corresponds, of coui'se, to the thickness of the intima, and 

 avarages in the anterior portion for several individuals 1.6//, in the 

 median portion 2.4/t. The average diameter of the poi'es for a 

 number of intestines was .5//. 



3. Summary of Structure. 



We have now considered the complete structure concerned directly 

 in the absorption of food, and have noted the changes in the cells 

 due to the process of moulting. To recapitulate, the apparatus 

 consists of an epithelium of large cells covered by a thin basement 

 membrane, which alone intervenes between the cell body and the 

 ccelome, and lined by a porous layer of chitin. On the mid-dorsal 

 wall of the anterior portion of the epithelium six longitudinal rows 

 of cells participate in the formation of a typhlosole. The cells have 

 no parietal limiting membranes, but are separated quite distinctly 

 from each other by intercellular supporting fibres. The cytoplasm 

 is alveolar. Between the alveoles course the intracellular fibres 

 from the intima to the basement membrane. Each cell contains a 

 large spherical nucleus alveolar in structure in the fresh condition, 

 filled with large granules of chromatin in the ' ' perfectly ' ' fixed 

 condition. At the luminal side of the cell the intracellular fibres 

 are parallel and are thickened so as to form a more or less rigid 

 palisade, from the intervals of which cytoplasm may be excluded. 

 The thickened ends of intracellular fibres serve, firstly, to preserve 



