142 BULLETIN OF THE BUREAU OF FISHERIES. 



The terminal intestine, or rectum, as it is sometimes called, runs backward a short 

 distance from its point of origin, parallel and almost in contact with the left anterior 

 retractor muscle of the byssus, then it turns obliquely upward to the anterior end of the 

 pericardial chamber. At this point it turns backward and passes directly through the 

 ventricle of the heart, continuing on in the mid line over the posterior adductor muscle 

 and terminating in an anus surrounded by dark-brown pigmented epithelium, situated 

 on the posterior surface of the muscle (fig. 120, TI, p. 141). 



In a mussel 80 mm. long, the mouth opening has a width of 7 mm., the oesophagus a 

 length of 12 mm., the stomach 13 mm., the direct intestine 32 mm., the recurrent intes- 

 tine 48 mm., and the terminal intestine 52 mm. 



HISTOLOGY. 



List (1902) has carefully worked out the microscopic structure of the labial palps 

 of Mytihis gallo provincial is, and since it is practically the same as that found in those of 

 Mytilus cdulis his description is followed. The entire upper surface is uniformly covered 

 with a thick layer of cilia. On the under surface cilia are also present, but they are dis- 

 tributed in small scattered clusters which are difficult to demonstrate (figs. 121 and 122, 

 opp. p. 142). 



The epithelial cells of the ridges are distinguished from those of the other parts of the 

 palp by their height. They are very small, cylindrical cells with a definitely streaked 

 cuticular border on which the cilia rest, and contain long, oval-shaped nuclei in which 

 are small chromatin granules. The cuticula is also characteristic of the epithelial cells 

 lining the smooth surface. At the base of the epithelial cells there are sometimes present 

 small, round cells which are perhaps ganglionic in nature. There are also some small 

 cells with peripherally running protoplasmic processes, the nuclei of which lie close to 

 the basal membrane. Whether or not special sense cells are present List was unable to 

 determine. 



In the epithelium of both the upper and lower surfaces single-celled glands of the 

 beaker type are present and are filled either with eosinophile granules or so-called mucin — 

 a slimy substance which stains strongly with mucin carmine, hsemalum, methyl green, 

 etc. Both kinds of cells are especially abundant at the ends of the ridges. 



Nerve fibers run over the surface of the palps in large numbers, and from these, side 

 branches arise to supply each ridge with a fiber (fig. 160, BuN, p. 173). 



The musculature of the palps is well developed, as would naturally be expected for an 

 organ which can perform such complicated movements. Large bundles of longitudinal 

 muscles lie just below the epithelium of the dorsal and ventral surfaces. They extend 

 from the base of the palps to the tip and are especially well developed on the under side. 

 Directly under the epithelium there are a few fine circular muscle fibers. There are 

 also fibers, somewhat better developed than these, which run through and across the 

 organ. Three sets of muscles are present on the rigid side of the palps. They consist 

 of (1) fine, circularly running fibers which lie just below the epithelium (2) bundles of 

 fibers which run from one ridge to the next, thus joining all of them together, and (3) 

 fibers which run from the base of the epithelial cells of the ridges to the epithelium of 

 the under side. 



The epithelium of the lips is continuous with the ridged epithelium of the palps 

 but differs from it by having taller ciliated cells. Below the epithelium of the lips 



