SUB-CLASSES LUCERNARIDA, GEAPTOLITID^, ETC. 81 



hollow rod, which no doubt served to strengthen the polypary, 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 Sei-tularians — namely, by means 

 of reproductive buds enclosed in horny capsules. Graptolites most 

 u.sually present themselves as beautiful silvery impressions, covering 

 the surface of the black shales of vai-ious parts of the Silurian system. 



Sub-class Hydrocorallin^. 



This name has recently been proposed by Professor Moseley 

 for two groups of marine animals which 

 produce a regular skeleton of carbonate 

 of lime, and which have, therefore, been 

 generally placed among the true Corals. 

 Professor Moseley, however, has shown 

 that the foi-ms 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 Millepores (Mille- 

 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 apertures 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- 

 ulse"), 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 

 are occupied in the living condition by 

 zooids (" gastrozobids "), which possess a 

 mouth and tentacles, whilst the smaller 

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



A % 



D 



Fig. 45 — A, Graptolites (Mon- 

 ograptus) priodoti, Broun, 

 pres?erved in relief : lateral 

 view slightly enlarged. B, 

 Dorsal view of a fragment 

 of the same species : con- 

 siderably enlarged. C, Front 

 view of a fragment of the 

 same, showing the mouths of 

 the liydrothecse : much en- 

 larged. D, Transverse sec- 

 tion of the same. All from 

 the Silurian. (Original.) 



