A TAXONOMIC STUDY ON PYROSOMA — METCALF AND HOPKINS. 199 



question of their specific distinctness by determining the chromosome 

 number. Our material, however, is not suitable for such study. 



This introductory discussion of the meaning of our taxonomic 

 distinctions has seemed necessary before entering upon the descrip- 

 tion of the different forms of Pyrosoma. We trust it has given the 

 reader an understanding of the difficulties that have confronted us 

 and will make him more kindly disposed toward our conclusions as 

 expressed in our classification. The results are unavoidably some- 



jca. 



Pig. i.— a colony of Pyrosoma atlanticum atlanticum; twice natural size. 

 After Ritter (1905). For significance of reference letters see Explanation 

 of Plates, p. 268. 



what vague and uncertain. The facts of structure of course are 

 clear, but the digestion of these phenomena and their expression in 

 a definitive classification is in considerable measure a matter of 



judgment. 



GENERAL DESCRIPTION. 



Pyrosoma has the form of a compact colony composed of numerous 

 individuals or ascidiozooids imbedded in a gelatinous tube which is 



Fig. 2.— A diagrammatic longitudinal section of a Pyrosoma colony; twice 



NATURAL SIZE. 



generally rather long, of somewhat uniform diameter, closed at one 

 end and open at the other (fig. 1). The zooids are placed with 

 their oral apertures to the outer surface of this tube, the atriopores 

 opening into the common axial colonial chamber. Thus the respi- 

 ratory current passes from the outside, through the zooids, into the 

 colonial tube, and thence to the exterior by way of the colonial 

 aperture. A longitudinal section through the entire colony (fig. 2) 

 reveals the form to best advantage. The cormus is nearly cylin- 



