160 ZOOLOGY 
The Gephyrea Achaeta include ten genera; of these Phas- 
colion and Phascolosoma occur in our seas, the former usually 
making its home in the shells of dead molluses, the latter 
living at the bottom of the sea half buried in sand. 
The largest genus of the class is Phymosoma; it 
Fic. 103.—Semi-diagrammatic view of the right half of the head of Phymosoma varians, 
seen from the inner surface. 
1. Mouth. 8. Brain. 
2. Lower ventral lip. 9. Circular nerves at base of ridges 
3. Oesophagus. bearing hooks. 
4, Portions of the coelom, seen in three 10. Sense organ at base of ridges bearing 
places. hooks. 
5. Blood sinus in lower lip corresponding 11. Rings of hooks. 
with 4 in Fig. 105. 12. Retractor muscle. 
5’, Blood sinus surrounding brain, and 138. Extensile collar. 
opening into 5”, the dorsal vessel. 14. Pit leading to brain. 
6. Skeletal tissue surrounding mouth. 15. Lophophore. 
7. Ventral nerve cord. 16. Eye-spot. 
contains twenty-eight species, which, with very few exceptions, 
are confined to the tropics, in many respects it resembles 
Phascolosoma, and, as its structure has been lately worked out, 
it will form the most convenient type for description. 
