ZooL.— Vol. III.] ROBERTSON— ASCORHIZA OCCIDENTALIS. lOI 



Viewed in optical section, the muscular plates are seen 

 passing from the base and extending through the stem into 

 the head (fig. i). At the proximal end of the stalk, the 

 muscle plates are crowded together somewhat irregularly, 

 but farther up they become more regularly disposed in 

 rings or segments. Just above the junction of the head 

 and stalk the irregular arrangement of the plates is very 

 marked. The reason for this becomes evident when it is 

 found that new plates are formed by a process of intercala- 

 tion. Each originates as a minute bud surrounded by a 

 delicate ectocyst (fig. 3, m. b.), pushing its way among its 

 neighbors and shoving them away from all sides of it. 

 Each muscle bud takes what shape it may conformably 

 with its surroundings. At first it is rounded or triangular; 

 then it becomes oblong, with its long axis at right angles 

 to the axis of the stem. As growth proceeds, it finally 

 becomes longer than broad, and takes its place in a defi- 

 nite segment. The muscle plates extend in a single layer 

 around the stem, and are arranged much as the bricks in 

 a wall. Each is lined with a layer of epithelial cells, within 

 which there extends a layer of muscle fibres from the upper 

 to the lower edge of the plate. The boundaries of the mus- 

 cle plates are as distinct as those between the zooecia of the 

 head, or base; and in no case do the muscle fibres extend 

 from one muscle plate to another. In cross-section (fig. 4), 

 the stalk is seen to possess, for the most part, only a single 

 layer of these muscular structures. They surround a cen- 

 tral cavity or lumen (/.), which is continuous with a cavity 

 in the head. 



The number of fibres in a muscle plate seems to vary, 

 the larger part of the interior being occupied, in some 

 instances, by mesodermal cells. In two instances, a num- 

 ber of sections in a series showed a rounded greenish body 

 {g. b.), reminding one of the remains of a degenerated 

 polypide. It was remarked above that the stalk is, for the 

 most part, one-layered. There is evidence, however, that 

 the muscle plates overlap to a certain extent, so that at 

 intervals there will appear in cross-section the lower part of 

 an upper plate lying inside of the boundary of a lower plate. 



