MUSEUM OF COMPARATIVE ZOOLOGY. 329 



of the zooids, and the constrictions upon it indicate the bodies of 

 the latter. Cy the deepening of these constrictions, each of the sinus 

 chambers, which are diverticula from the body cavity of the solitary 

 form, becomes divided up to form the body cavities of the zooids 

 upon one side of the chain. 



The central partition gives rise, near one edge, to a row of bud like 

 protrusions upon each side, which become the branchial and digestive 

 organs of the zooids of each side of the chain ; while a similar 

 double row, upon the other edge, give rise to the ganglia. It is prob- 

 able that the cavities of the branchial sacs and gauglia originate as 

 lateral diverticula from the tubular chamber of the partition into 

 these buds; but this, for reasons which will be stated presently, could 

 not be determined with certainty. The club-shaped organs within the 

 sinus chambers become divided up into single rows of eggs, one of 

 which passes into the body cavity of each zooid at a very early 

 period of development. 



It will be seen from this account that, in Salpa, as well as in Pyroso- 

 ma, " gemmation takes place, not, as in so many of the lower animals 

 [e. g. the Hydrozoa and Polyzoa], by the outgrowth of a process of 

 the body wail whose primarily wholly indifferent parietes become 

 differentiated into the organs of the bud ; but, from the first, several 

 components, derived from as many distinct parts of the parental organ- 

 ism, are distinguishable in it, and each component is the source of cer- 

 tain parts of the new being, and of these only." * While these changes 

 are in progress the tube lengthens, so that it at length encircles the 

 nucleus, as shown in Figs. 1 and 2, u. The constrictions upon its 

 surface deepen, and the wall protrudes between them, and each is soon 

 seen to mark off, on each side of the stolon, the body of a young Salpa, 

 and these soon become large enough to be visible to the unaided eye. 



They do not increase in size gradually, from one end of the tube to 

 the other, but develop in sets of from thirty to fifty each, and the de- 

 velopment of all which are embraced within a set progresses uniformly. 

 There are usually three of these sets upon the tube of an adult soli- 

 tary Salpa, but sometimes there are four, two, or only one. 



* Huxley, on the Anatomy and Development of Pyrosoma (p. 211). 



I hr.ve taken the liberty of slightly changing this quotation, since the portion in 

 brackets reads, in the original, "the Hydrozoa and Polyzoa, or Salpa and Clavelina 

 among the Ascidians." 



