58 THE VOYAGE OF H.M.S. CHALLENGER. 



alongside the future nucleus, and probably belonging to the hypoblast, become filled up 

 with oil globules to form a mass of nutrient material, — the " elseoblast," — which is used 

 up later on in the development. Many suggestions have been made by the various 

 writers as to the homology of this elseoblast. The most probable is that it is the 

 disappearing rudiment of the tail found in the larval condition of most Ascidians. 

 The free-swimming tailed larval stage is not found in the life-history of Salpa — the 

 development being direct. 



The testis, which arrives at maturity in the aggregated Salpa only after the 

 embryo has been got rid of, is a system of tubules ramifying on the outer surface of the 

 visceral mass and opening into the peribranchial cavity. 



The embryo produced sexually by the aggregated Salpa becomes a solitary Salpa, 

 and this while still young develops a stolon as a projection on the right side of the 

 body ventraUy and close to the heart. This stolon is an outgrowth of the body-wall, 

 containing prolongations of all the more important systems of the body — branchial sac. 



Fig. 8. — Posterior part of solitary form of Salpa democratica-mucronata, showing a well-developed chain. — (From the Encycloix-edia 



Britannica, 9th ed. ) 

 ;iem. young aggregated Salpcc forming the chain ; m. mxiscle band of mantle ; St. stolon ; (. test ; vise, visceral mass. 



pericardium, blood-sinuses of mantle, elseoblast (hypoblast cells ?), and probably nervous 

 system. As the stolon elongates it becomes segmented into pieces, each of which 

 develops eventually into the body of an aggregated Salpa (see Fig. 8). After the 

 solitary Salpa has become fully developed, the chain produced by the stolon is set free 

 in sections, each section being composed of a number of aggregated Salpae at about the 

 same stage of development. In most cases the Salpse in a chain are placed in a double 

 row, and alternate so that each one touches the bodies of four of its fellows. It is 

 joined to each of these neighbours by processes of its body-wall composed of the mantle 

 covered by a layer of ectoderm. AVhen the chain is still young the test is thin, and 

 these processes, which join the bodies of adjacent Salpw, are relatively long, and keep 

 the members of the chain far apart ; but as they grow older, and their tests thicken, 

 the Salpw become pushed farther apart, and the joining processes are finally com- 

 pletely embedded in the tests, and the chains are then ready to break Up on the 

 slightest touch, and the Salpx lead the rest of their existence in a separate condition. 



