'Calkins. — Hydroids. 
PLATE 4. 
Tig. 18. Obelia griffini. a. Hydrocaulus. Nat. size. b. Hydrotheca and 
hydranth. X 80. c. Gonosome. X 80. 
Tig. 19. Campanularia exigua. Nat. size. a. Hydrocaulus. X 18. b. Hydro¬ 
theca and hydranth. X 80. 
Tig. 20. Calicella syringa. Nat. size. a. Hydrocaulus. X 18. b. Hydrotheca 
and hydranth; operculum thrown out. X 80. c. Hydrotheca and 
hydranth; operculum drawn in. X 80. 
Fig. 21. Sertularella tricuspidata. Nat. size. a. Hydrocaulus. X 18. b. 
Gonosome. X 80. c. Hydrotheca. X 80. 
Tig. 22. Sertularella conica. Nat. size. a. Hydrocaulus. X18. b. Hydro¬ 
theca. X 80. 
Tig. 23. Obelia plicata. Nat. size. a. Hydrotheca and hydranth. X 80. 
