HYDROZOA. GRAPTOLITOIDEA 



61 



when perfect, are pear-shaped, and resemble the gonangia 

 of living Calyptoblastea (see fig. 12, 10); they are not 

 joined to the hydrothecse, but come off at right angles to 

 them along the middle line of the sides of the polypary. 

 The earliest condition of the graptolite at present known 

 is the sicula (fig. 18 A) ; this probably is developed within 

 the sac-like bodies if they are, as suggested, really 

 gonangia. 



Fig. 17. 



Fig. 16. 



Fig. 16. Diplograptus with sacs resembling gonangia. (After Hall.) 



Natural size. 

 Fig. 17. Didymograptus v-fractus, Arenig Beds. Early part of the 



polypary. (After Elles.) s, sicula ; c, crossing-canal ; 1, first hydro- 



theca ; 2, second hydrotheca. x 5. 



The mode of development of graptolites has been 

 studied in several genera. The sicula is usually more or 

 less clearly exposed at the proximal end of the polypary 

 (fig. 13 c) ; it is a hollow cone (fig. 18 A) open at the base, 

 and consists of two parts — the pointed or apical end with 



