SUB-CLASSES LUCERNARIDA, GEAPTOLITID.'E, ETC. 81 



hoUow rod, which no doubt served to strengthen the polj'pary, and 

 "which is often prolonged beyond one or both ends of the polypary in 

 a naked state. There is also good evidence that the reproductive 

 process in the Graptolites was carried on in manner somewhat 

 similar to what is seen in the living Sertularians — namely, by means 

 of reproductive buds enclosed in horny capsules. Graptolites most 

 usually present themselves as beautiful silvery impressions, covering 

 the surface of the black shales of various parts of the Silurian system. 



B 



Sub-class IIydrocoeallinjs. 



This name has recently been proposed by Pi-o'cssor Moseley 

 for two groups of marine animals which 

 produce a regular skeleton of carbonate 

 of lime, and which have, tlierefore, been 

 generally placed among the true Corals. 

 Professor Moseley, however, has shown 

 that the foiTus in question are really re- 

 ferable to the Hydrozoa, though they are 

 peculiar, as regards the members of this 

 class, in their power of secreting a regu- 

 lar calcareous skeleton. The best known 

 and most familiar of the animals placed 

 in this group are the Millejjores (Jfillc- 

 pora), the stony skeletons of which con- 

 tribute so largely to the formation of 

 coral-reefs in the West Indies and Pacific, 

 The skeleton of Millepora is an expanded, 

 more or less branched, calcareous mass, 

 studded with minute aj)ertures of two 

 sizes, the mouths of differently - sized 



tubes. These tubes are divided into '■ *• '*'■ 



chambers by a number of complete 

 transverse partitions (the so-called " tab- 

 uke"), and only the surface-layer of the 

 mass is to be regarded as actually alive. 

 The general calcareous tissue of the skel- 

 eton is open and spongy, and allows of a 

 free communication between the different 

 tubes. The larger tubes of the skeleton 

 !ire occu])ied iu the living condition bj' 

 zooids ("gastrozoijids"), which jrassess a 

 mouth and tentacles, whilst the smaller 

 tubes are tenanted by smaller zooids (" dactylozooids "), which possess 



ig. 40 —K, G-mptnliles (.Voji- 

 ograpti(s) prwd'iu, Broim, 

 pi'escrvpd in relief : lateral 

 view slightly enlargefl. B, 

 Dorsal view of a fragment 

 of the same species : con- 

 sMerably enlarged. C, Front 

 view of a fragment of the 

 same, showing the nm'iths of 

 the liydrntheeie : much en- 

 larged. D, Tjaiisvei'se sec- 

 tion of the same. All from 

 the Silurian. (Original.) 



