402 



TUNICATA. 



(wandering mesoderm-cells) become arranged in a very characteristic 

 way in Pyrosoma. They form rows, which are connected in such a 

 way as to produce hexagonal areas (Figs. 195, 196, and 198, ms'). In 

 later stages the processes of the ectoderm-cells which traverse the 

 test are again withdrawn. The hexagonal pattern which is produced 

 by the arrangement of the mesoderm-cells is, however, retained for 

 some time. 



FIG. 196. Two later outogenetic stages of the tetrazooid colony of Pyrosoma (after 

 KOWALEVSKY). c, Cyathozooid ; cl, atrial aperture of the Cyathozooid, cloacal 

 aperture of the young colony ; d, dorsal elongated cell-mass ; el, elaeoblast ; en, 

 endostyle ; i, inhalent orifice ; I, lens-shaped cell-mass ; n, central nervous system ; 

 r, languets ; s, connective strand between the Cyathozooid and the first Ascidio- 

 zooid ; s' and s", connective strands between the separate Ascidiozooids ; z, remains 

 of the cell-zone. 



According to SALENSKY, the ectoderm of the Cyathozooid is also 

 capable of secreting cellulose substance on its inner surface, and this 

 accounts for the fact that the ground-substance of the mesenchyme 

 which fills a part of the primary body-cavity is cellulose. Through 

 the development of this intermediate layer, the alimentary canal 

 becomes removed a considerable distance from the ectoderm of the 

 Cyathozooid. 



The simplicity of structure of the Cyathozooid is explicable when 

 we consider that its sole function is to give rise by budding to the 

 four primary Ascidiozooids. As soon as these have attained a certain 



