246 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



be left out of consideration in connexion with these degenerating organs. 

 The genera dealt with are Cydosalpa, Brooksia, Apstehiia, Salpa, Ritteria, 

 Jasis, Thalia, Theti/s, Pegea, and Traudedtia ; a chart of relationship 

 is given. The major divisions are Dolichod^ea, Sphaerod^a, and Cir- 



cs'. 



rSH 



^=Z^=M£^i 



J^^^iOlm^.^ 



Apsteinia asymvietrica, an embryo 2 • 5 mm. long, viewed from 



the right side. 



pi., "placenta"; el., eleoblast ; a.o., atrial opening; grj., ganglion; 

 c.f., ciliated funnel ; m., mouth ; the other letters refer to muscles. 



coda^i. In the group Salpidae hybridization is unknown. It is probable 

 that the Doliolidaj arose from somewhat Pyrosoma-\\kQ ancestors, l»y 

 giving up the compact colonial form, and that from Doliolum-Vik^ 

 ancestors arose the Salpidte. J. A. T. 



Taxonomic Study of Pyrosoma. — Maynard M. Metcalf and 

 HoYT S. Hopkins {Bull. U.S. Nat. Museum, 1919, 100, 195-275, 

 21 pis.). A fine study based on the collections in the U.S. National 

 Museum. The family Pyrosomidse is generally regarded as containing 

 but one genus, Pyrosoma, but the groups Pyrosomata anibulata and 

 Pyrosomata fixata might be regarded as separate genera. The authors 

 have studied 213 colonies, comprising 13 species and varieties, 

 including all but 4 of the forms of Pyrosoma hitherto described, and 

 6 undescribed forms. Some of the forms are probably transient, for it 

 seems that the genus is in flux. Within the two subgenera, which are 

 sharply distinct, there are groups of forms with much intergradation, 

 and others which diverge. It is difficult to lay down the law about 

 species until we know whether mutation is continuing at present. The 

 conditions suggest that interbreeding, especially in the atlanticum series, 



