1 70 



THE INVERTEBRATA 



Graptolites are represented in the earliest fossiliferous rocks, the 

 Cambrian, Ordovician and Silurian. Though we know nothing of 

 their soft parts, the exoskeleton was horny or chitinous and so may be 

 well preserved. It resembles in general development that of the 

 colonies of the Calyptoblastea, in that it was produced round the 

 polyps to form definite hydrothecae. The graptolites, however, differ 

 from calyptoblast hydroids because new individuals have the appear- 

 ance of being budded off directly from older ones rather than from 



m. 



h.2 



)ThA 



A « C 



^'^- -^- Fig. 1.7. 



Fig. 126. A. Young colony of Monograptus, showing prosicula (/)), meta- 

 sicula (m) and developing hydrothecae i and 2, with growth-lines. After 

 Kraft. X15. B. More mature colony of Mowo^rop/M^. X2-5. C. Early part 

 of the colony of Climacograptus, showing sicula {s) partly enclosed by the 

 early hydrothecae. After Wim an. X15. 



Fig. 127. A, Didymograptus \-fractus. Ordovician. Early part of the poly- 

 pary. After Elles. si. sicula ; cr.c. crossing-canal ; i, first hydrotheca ; 2, second 

 hydrotheca. X5. B, Tetragraptus similis. Lower Ordovician. Young form 

 with virgula and disc. After Ruedemann. x 4. 



a common coenosarc. Each colony originates from a conical body 

 called a sicula^ the exoskeleton of the first formed individual, consist- 

 ing of the pro- and meta-sicula (Fig. 126 A). From the side of the 

 metasicula a bud is formed, which develops into the first hydrotheca, 

 and from this is produced the second, and so on. In this way a linear 

 series of polyps is produced which are arranged in a slender lamella 

 (stipe) y the hydrothecae being in contact and the cavity of the colony 

 being continuous. This is the simplest arrangement, and is seen in 

 Monograptus (Fig. 126 A and B) where the hydrothecae are all on one 



