232 



RITTER 



ing the antero-posterior diameter somewhat longest. Very regular in 

 outline and unbroken in surface. Test thin, entirely covered by a thin 

 uniform layer of closely adhering sand grains, clinging to the test sub- 

 stance itself and not to processes growing from it (p1. xxviii, fig. lo). 

 Frequently several individuals cling together by their peduncles, but 

 this is apparently due to the tangling of the slender peduncles with 

 one another, and not to actual fusion. Peduncle frequently broken off 

 at or near body. 



MEASUREMENTS. 



Usually somewhat laterally compressed. 



Color uniform gray from the sand covering the surface ; test, except 

 for the sand, very transparent. A small area of chocolate color at and 

 around the branchial orifices. 



Mantle very thin, separating readily from the test. A large num- 

 ber of small ectodermal processes projecting from its outer surface 

 toward the test, not, however, reaching into the test so far as ob- 

 served (p1. xxviii, figs. 19^ and 19^). 



Musculature not highly developed. Longitudinal muscle fibers 

 grouped into very definite, regularly spaced bundles radiating from 

 the two siphons. Between the siphons a band of closely-placed par- 

 allel bundles reaching entirely across from one side of the animal to 

 the other. Circular fibers also disposed in bundles^ smaller and less 

 regular, however, than the longitudinal bundles. Circular fibers out- 

 side the longitudinal ones ; a thin but distinct layer of mesenchymatous 

 tissue between the two. 



Branchial apparatus. — Siphons very short and inconspicuous, al- 

 most invisible to surface views, situated at the anterior end, nearly as 

 far apart as the width of the animal; branchial with six (occasionally 

 fewer) small lobes ; atrial with four lobes. Branchial tentacles 

 twelve to fourteen in number, branched, but not deeply or profusely. 



Hypophysis mouth horseshoe-shaped, broader than long, opening 

 forward, somewhat to the left, horns not coiled (p1. xxviii, fig. 19). 



