A TAXONOMIC STUDY ON PYROSOMA — METCALF AND HOPKINS. 211 



follicle membrane. Later, mesenchyme cells enter this secreted layer, 

 forming the so-called test-cells. Kowalevsky thought that the 

 atrium of the cyathozooid became the colonial cavity of the colony. 

 According to Salensky, the cyathozooid degenerates and the colonial 

 aperture has an independent origin. In our colonies we find em- 

 bryos in which the atrium of the cyathozooid is completely overgrown 

 by a layer of cellulose. After the four buds become more separated by 

 the constrictions of the stolon each one enlarges through growth and 

 then turns through an angle of 90° so as to take up a position with 

 its dorsal side toward the cloaca of the cyathozooid. While the aper- 

 ture of the colony is developing in this region each ascidiozooid sends 

 out dorsally two ectodermal processes, the test vessels, into the test. 

 At about this stage, or perhaps somewhat earlier, the little tetrazooid 

 colony is set free from the cloaca of the parent zooid, passing out of 

 the colony, and commences an independent existence. A mouth 

 and atrial aperture form in connection with each zooid, the latter 

 communicating with the exterior through the colony chamber. 

 The primary ascidiozooids are now capable of growing independently 

 and of producing buds for the further development of the colony. 



We shall not enter into detail in describing the organogeny of the 

 zooids. In the four primary ascidiozooids the processes are some- 

 what easier to interpret than are those in the later formed, secondary 

 zooids. The peribranchial tubes of the stolon segment into four 

 pairs of peribranchial pouches. These, growing together posteriorly 

 in each zooid, form the atrium, the aperture of which appears later 

 from an invagination of the outer ectoderm. The enteric tube like- 

 wise becomes constricted, but not completely so until quite late. 

 From the enlarging pharynx cavity are formed several organs: the 

 definitive endostyle, from an endostyle rudiment; a digestive tract, 

 by evagination of the posterior (lower) side of the pharynx wall; the 

 branchial chamber; and later the oral chamber and mouth, partly 

 by the infolding of the outer ectoderm of this region. Gill-slits 

 appear as elongated pores, always lying transverse to the endostyle; 

 these are crossed by folds formed later across their inner surface 

 (longitudinal bars). The nervous system arises from the ectoderm 

 by invagination, as previously mentioned. The heart develops anew 

 from a right, mesodermal cell strand very much as it does in the 

 cyathozooid. The remaining mesoderm passes into the stolon to give 

 rise to genital and eleoblast tissue. 



In the secondary ascidiozooids the peribranchial pouches are de- 

 rived, according to Seeliger, from the genital strand (mesoderm). 

 According to this same author the neural rudiment also comes from 

 mesoderm. But Neumann (1912) maintains that the peribranchial 

 pouches and the nervous system arise from corresponding stolon- 

 rudiments derived from the parent zooid, hence they are probably 



