274 University of California Publications in Zoology. [Vol. 4 



sequently the organs of one side of the animal are visible. When 

 moving very rapidly it often stands upright with the aboral end 

 directed upward or sometimes downward. As conditions become 

 more unfavorable, movement of translation ceases, and the animal 

 lies on its side, remaining for some time in an uncontracted state, 

 that is, the atrial cavities remain open ; as conditions become still 

 more unfavorable, quivering motions are set up in the bundle of 

 fibers extending between the pyriform organ and the retractile 

 disk, and finally the whole body contracts into a mass in which 

 both organs and atria are indistinguishable. 



Examination of the kelp containing young stages of colony 

 formation of M. villosa soon revealed many instances of the fixa- 

 tion of the larva. In the earliest stage observed (pi. 17, fig. 24a) 

 the shell is seen to have opened along its anterior edge and to 

 have flattened over its contents, its edges being closely applied to 

 the surface of the kelp and the shell being slightly raised in the 

 middle. At first the larval organs form an indiscriminate mass, 

 apparently undifferentiated, retaining the shape of the flattened 

 shell, but smaller than it. In the cellular mass, lines of differ- 

 entiation soon appear, and a border forms inclosing two cell 

 masses, a and b, and between them a third, c (fig. 24a). The 

 interior is thus divided into three distinct masses of cells and into 

 two chambers, a and b, which constitute the first two zooecia of 

 the new colony. The masses of cells in fig. 24a become trans- 

 formed into the first two polypides, a,' and b' (fig. 24b). On the 

 border of the young zooecia four or five long bifid spines extend 

 outward while on the wall of the aperture just below the oper- 

 culum, one or two minute frontal spinules, become visible. The 

 twin polypides seem to advance with equal rates of growth. Be- 

 tween the apices of the zooecia the cell mass, c, becomes the 

 fundament of the third zooecium (fig. 25) with its polypide. 

 On each side of the third zooecium another forms, all being fur- 

 nished with extremely long branched spines, the marginal zooecia 

 having four or five, the intermediate one a smaller number. 

 A later stage shows seven zocpcia (pi. 16, fig. 23), and so on 

 until the colony numbers hundreds of zooecia. Instances are 

 found in abundance where the shell remains in situ until after the 

 branched spines have formed around the margin of the primary 



