588 MATNARD M. METCALF, 



proportional to the size of the cells themselves, therefore the meas- 

 urements of the nuclei give a fair criterion for comparison. 



The larger and smaller species of a genus usually have cells of 

 approximately the same size, and the same thing holds true, with 

 few exceptions, even if we compare representatives of diiferent 

 genera or different families. 



Measurements of cells and comparisons were made for the 

 following species: 



JBostricJiohranchus molguloides (30 mm X 25 mm X 22 mm) ^). 

 Molgula manhattensis (20 mm X 15 mm X ^^ mm), 

 M. arenosa (V) (11 mm X 16 mm X 12 mm), 

 Cynthia papulosa (64 mm X 32 mm X 35 mm), 

 C. pyriformis (60 mm X 27 mm X 24 mm), 

 C. carnea (4 mm X ''^ ^^^^ X 6 mm), 

 Styela gyrosa (45 mm X 32 mm X 24 mm), 

 S. aggregata var. americana (?) (20 mm X I'l^ iii^ X 13 mm), 

 Polycarpa varians (56 mm X 28 mm X 23 mm), 

 P. glomerata (7,5 mm X 4 mm X 3,75 mm), 

 Phallusia mammillata (85 mm X 48 mm X 43 mm), 

 Rhopalaea neapolitana (50 mm X 24 mm X 20 mm), 

 Clavelina rissoana (27 mm X 7 mm X 5,5 mm), 

 Ecteinascidia turbinata (19 mm X 8 mm X 6,5 mm). 



In all these forms the cells measured are nearly of the same 

 size, except in the two Molgulas and the two large Cynthias (C pa- 

 pulosa and C. pyriformis) in which the cells are somewhat smaller 

 than in the other forms measured, and in Rhopalaea neapolitana in 

 which the cells are nearly one half larger. 



Reference to the figures on Plates 34 to 38 shows the same 

 relations graphically. Of course, the various degrees of magnification 

 of the different figures should be taken into account. 



2) The Lens of the Eye in the Tadpole of Ecteinascidia 

 turbinata Herdman. 



The lens of the eye in the tadpoles of most Ascidians is formed 

 by the metamorphosis of a single cell which migrates from among 

 those of the epithelial cells of the sensory vesicle which are destined 

 to form the rod-cells of the eye. It may be of some interest to re- 



1) These measurements are the dimensions of the body height 

 length (antero -posterior) and breadth. 



