vou 2, pt. 2.] A TAXONOMIC STUDY OF THE SALPIDAE METCALP. 137 



protoplasm reacts much more strongly with hematoxylin, giving a deep stain, while the 

 protoplasm of the rod-cells stains very weakly. These characters, and especially the 

 great thickness of their cell walls, distinguish the rod-cells of the eye, and of the two lat- 

 eral masses in the ganglion, from any others of the nerve cells. We must, then, regard 

 these two lateral masses of rod-like cells that are found in the ganglion as imperfect or 

 degenerate eyes, bearing the same relation to the larger eye of this species as the smaller 

 dorsal eyes of the chain Cyclosalpa pinnata do to the large, unpaired eye of that species. 

 These structures just described form a connecting link between the smaller eyes found 

 in Cyclosalpa pinnata [see fig. 9, p. 23] and Salpa cylindrica [see figs. 87, p. 98, and 89], 

 which are undoubted optic organs, and other structures found in Salpa hexagona [see 

 figs. 45, p. 70, and 47], Salpa costata-tilesii [see fig. 118, p. 126], and Salpa cordiformis- 

 zonaria [see fig. 103, p. 108], which are so different from the typical eye that one would 

 not readily recognize their true character. 



OCCURRENCE OP PEGEA CONFEDERATA. 



The aggregated form of Pegea confederata is one of the most 

 abundant of the Salpa group. In the collections made by the United 

 States Bureau of Fisheries steamer Albatross in the Philippine waters, 

 it was the most abundant of all the Salpas, and in the United States 

 National Museum collections made since 1875 it is very abundant. 

 In all these collections there are, on the other hand, very few repre- 

 sentatives of the solitary form of this species except numerous 

 embryos from 5 mm. to 35 mm. long. There are five individuals, 

 one 38 mm. long, two 40 mm. long, and two 42 mm. long, in which 

 the eleoblast is absorbed. All the other specimens of the solitary 

 form, even the few which are of large size (2-3 J cm. in length), 

 show the eleoblast large and prominent and are evidently embryos 

 recently set free. The larger individuals mentioned above may 

 well be called adult, for all five of them show stolons, two of these 

 showing the aggregated zooids 1^ mm. in length. These five solitary 

 individuals were collected at the surface by the steamer Albatross, 

 one in July at station 4037, Hawaiian Islands, three from Philippine 

 waters in February and in September, and one from station 1098, 

 off the coast of New England. 



This scarcity of the solitary form of Pegea confederata is a general 

 condition in collections of Salpidae. 



The Challenger Expedition (Herdman, 1888) found five aggre- 

 gated zooids of the form bicaudata in one haul in Philippine waters, 

 and no solitary specimens. They also found, in the south Pacific, 

 the test of one large individual so degenerate that it was impossible 

 to distinguish whether it belonged to a solitary or an aggregated 

 individual. 



The Plankton Expedition (Apstein, 1894, b) found, in the West 

 Indies and the Gulf Stream, in the late summer and fall, 793 individuals 

 of Pegea confederata, of which 437 were aggregated zooids and 356 

 were solitary forms including embryos. No indication is given in 

 Apstein's report of the number of adult solitary individuals, if any 

 such were present. 



