vou2,pt. 2.] A TAXONOMIC STUDY OF THE SALPIDAE METCALF. 67 



The rod-cells (fig. 40) are degenerate angular cells with irregularly 

 thickened walls, as in the solitary Salpa fusiformis (fig. 79, p. 92). 

 Below the main mass of the horseshoe, on each side, is a secondary 

 mass (eq) of similar irregular rod-cells with no associated pigment. 



There are a pair of conical, denticulate protuberances (fig. 35) 

 from the postero-lateral angles of the body, somewhat similar to 

 those of TJialia longicauda (fig. 1 13, p. 120) and Thetis vagina (fig. 

 114, p. 122). Into the base of each runs a tube of mantle epithelium. 



RITTERIA HEXAGONA, aggregated form. 



The aggregated zooids of this species (Gg. 41) are quite strongly 

 asymmetrical, but the asymmetry affects chiefly the test, which has 

 an angular protuberance at one side of the pos- 

 terior end of the body. 



As in the solitary form, the muscle bands 

 are broad and strong, and the test very firm. 

 There are six body muscles, continuous across 

 the dorsal mid line, but widely interrupted ven- 

 trally. The intermediate muscle (i. m., fig. 42) 

 shows a condition which aids us in interpreting 

 the relations in the solitary form. There is a 

 broad band (i. m.) continuous across the mid- 

 dorsal line, and branching obliquely backward 

 from this on each side is a narrower band of 

 muscle (i. m. 2 ), whose posterior end approaches 

 but does not touch the dorsal region of body 

 muscle I. These oblique bands are apparently 

 the intermediate muscles. The broader band, 

 on the other hand, which is continuous across the 

 dorsal mid line, is the apparent homolog of the 

 muscle which in the solitary Ritteria hexagona we called the interme- 

 diate muscle. Apparently, in the aggregated zooids of this species, 

 the intermediate muscle is double, the posterior division being oblique 

 and discontinuous dorsally, the anterior division lying more vertical 

 and being continuous across the dorsal line. In a number of the 

 more aberrant species of Salpidae the intermediate muscle is double. 



The oral retractor muscle on each side is very short and broad 

 (fig. 42). It bifurcates in front, the dorsal branch being continued 

 into the first sphincter of the upper lip. In the upper lip are a second 

 narrow sphincter and "a third very broad. The lower end of the 

 second sphincter abuts upon, but does not fuse with, the third 

 sphincter; this third sphincter similarly abuts upon the oral re- 

 tractor muscle without fusing with it. The ventral branch of the 

 retractor muscle is continued into the very broad third sphincter of 

 the lower lip. There are two other sphincter muscles in the lower 



UKJ 



Fig. 41.— Ritteria hexago 

 na, aggregated zooid, 

 dorsal view. x 2 diam- 

 ETERS. From Apstein 

 (1894,6). 



