Brooks.] 196 LNovember 17, 



salpa is the female, and the chain salpa simply the male, which does 

 not reproduce, but simply serves to fertilize and nourish the egg, so 

 that we have, not an alternation of generations, but a very remarka- 

 ble difference in the form and mode of origin of the two sexes. 



The tube or stolon which is to form the chain first appears as a 

 protrusion or diverticulum from the outer or muscular tunic of the 

 solitary salpa, directly opposite the heart; this protrusion rapidly 

 increases in length, and soon presents the form of a long tube closed 

 at its distal end, projecting into the test, and with its cavity in direct 

 connection with the cavity of the sinus system (the body cavity) of 

 the solitary salpa, so that the blood of the latter enters and circu- 

 lates freely within it. (Figure X.) 



A second tube with very thick walls and a very narrow cavity 

 now grows out from the pericardium, crosses the sinus and penetrates 

 the cavity of the outer tube almost to its tip or blind end, and soon 

 becomes flattened and its edges unite with the walls of the outer 

 tube, which thus becomes divided into two chambers, which are en- 

 tirely separate from each other except at the tip. The blood now 

 passes into one of these chambers at its base, and is driven up to the 

 blind end where it passes around the partition, back through the other 

 chamber to the sinus of the parent. It is of course unnecessary to 

 state that when the circulation of the parent is reversed that of the 

 stolon changes also. 



By the formation of the partition above described the tube is di- 

 vided longitudinally into halves, and each half is destined to "be con- 

 verted into the series of" zooids" on one side of the chain. The outer 

 wall of the tube, which has been shown to be part of the muscular 

 tunic of the parent, becomes the muscular tunics of the "zooids"; the 

 chambers, which are continuous with the sinus system of the parent, 

 form the body cavities or sinus systems of the "zooids," and the central 

 tube, which is a prolongation of the pericardium of the parent, forms 

 the nervous, digestive and branchial organs of the " zooids " of the 

 chain. It is probable that the cavity of this inner tube gives rise to 

 lateral diverticula, which form the cavities of the digestive organs and 

 branchial sac of the young, but this point could not be determined 

 with certainty, nor could any connection between the cavity of this 

 inner tube and any of the cavities of the parent be discovered. 



Before the tube becomes differentiated into the organs of the "zo- 

 oids," in fact, before there are any indications that the tube is to give 

 rise to the chain, two new organs are formed, one in each of the sinus 



