484 TUNICATA. 



3. Pyrosoma. 



Since the development of the four primary Ascidiozooids of the 

 Pyrosoma colony is closely connected with that of the Cyathozooid, 

 and takes place almost entirely within the period of embryonic life, 

 it has already been described above. It still remains for us to 

 describe the process of budding by which the other individuals 

 found in the adult colony are produced. These processes have been 

 described by Huxley (No. 72) and Kowalevsky (No. 71), and later 

 by Juliet (No. 73) and Seeligee (No. 76). The first development 

 of the proliferating stolon has also recently received some attention 

 from Salensky (No. 74). The chief point which we shall have to 

 consider is the origin of the primary organs in the proliferating stolon. 

 The further development of the zooids agrees so closely with that of 

 the four primary Ascidiozooids described above that we need only 

 refer to it briefly. The following account is based mainly on the 

 detailed observations made bv Seeligee. 



A. Development of the Proliferating Stolon. 



The individual zooids of Pyrosoma start budding very early, even 

 at a time when their individual independence is not fully attained, as 

 they are still connected with the stolon from which they were pro- 

 duced. In such a stolon (Figs. 253 and 267) the individuals increase 

 in size from its proximal to its distal end, those lying furthest from 

 the parent (Fig. 253 III) being the most developed aud exhibiting 

 the almost perfect organisation of a Pyrosoma zooid and the first 

 rudiments of the future proliferating stolon. 



The first indication of this organ is an outgrowth . (d) of the 

 branchial or pharyngeal sac directed toward the ectoderm ; this lies 

 at the posterior end of the endostyle (es), ventrally to the heart (hz), 

 and appears surrounded by elaeoblast-tissue (eh). This structure is 

 known as the entoderm-tube, the entoderm-process, or, on account of its 

 close relation to the endostyle of the parent, the endostyle-process. It 

 seems probable that the stolon in every case is nothing more than the 

 remains of the entodermal tube which connects the entoderm-sacs of 

 two neighbouring buds (Fig. 253, v). 



Another element which enters into the formation of the prolifer- 

 ating stolon is a group of closely crowded mesoderm-cells (ms) 

 embedded in the elaeoblast. This must be regarded as the rudiment 

 of tin (jnutal strand of the budding stock. Even earlier one or more 



