DEVELOPMENT OF THE DUACIIIOPODA 



249 



consisting of a central cavity, or mesenteron, connecting on 

 each side with the body cavity. 



The Xcoembri/o, represented by the trochosphere and seg- 

 mented, cihated, cephaluhx stiiges, has Ijeen more fully ob- 

 served than any of the preceding. The first advance from 

 the completed gastrula is in the separation of the mesenteron 

 from the body cavity, and the division of the organism into 

 two segments or lobes, the cephalic and caudal (figure 89). 

 Later a third or thoracic segment is developed and carries 

 four bundles of stiff barbed setse (figure 90). The cephalic 

 and caudal lobes are densely ciliated. During the subse- 

 quent cephalula period two eyes, then two others, appear in 

 Cistella, and at the same time the dorsal and ventral sides 

 of tlie thoracic segment become extended over the caudal, 

 and are progressively defined as two lobes (figures 89-93, 

 108, 109). 



89 90 91 92 



Cistella neapolitana Scacchi. 



Figure 89. — Neoembryo ; embryo of two segments. 



Figure 90. — Neoembryo ; cephalula ; ventral side ; showing cephalic, thoracic, 

 and caudal segments, eye-spots, and bundles of setae. (Figures 89 and 90, after 

 Kovalevski.) 



Figure 91. — Neoembryo; lateral view of completed cephalula stage; show- 

 ing extent of dorsal (rf) and ventral (i;) mantle lobes, and umbrella-like cephalic 

 segment. 



FioirRE 92. — Neoembryo; same stage; ventral view. (Figures 91 and 92, 

 after Shipley.) 



Terebratulina has a tuft of bristles on the top of the 

 cephalic segment. In Laoazella the bundles of setse are 

 absent, and the head is more distinctly differentiated from 

 the anterior segment than in Cistella. The closing cepha- 

 lula stage in Cistella has an umbrella-like expansion of the 



