HYDROZOA. CALYPTOBLASTEA 55 



may be in rows or placed in various positions on the 

 hydrocaulus. In Plumularia, Aglaophenia, etc., they 

 form a single row ; in Sertularia, etc. there are two rows 

 placed on opposite sides of the branches. Sometimes the 

 hydrothecse are close together, but more usually they are 

 separated. 



In the Plumulariidse there are, in addition to the 

 ordinary polyps, others which are solid and tentacle-like ; 

 they are usually provided with nematocysts and are called 

 nematophores ; each one is placed in a hydrotheca. 



Although the Calyptoblastea possess a well-developed 

 chitinous skeleton, yet, with the exception of a form found 

 in the Pleistocene, they are not definitely known to occur 

 as fossils. In the Lower Palaeozoic, however, there are 

 organisms (usually termed 'dendroid graptolites ') which 

 present considerable resemblance to the Calyptoblastic 

 hydroids ; the best known are Dendrograptus, Ptilo- 

 graptus, Dictyonema, and Callograptus. These are usually 

 much branched and tree-like, and are fixed by a root-like 

 structure ; hydrothecse occur, but no virgula is found. 

 Transverse sections of Dictyonema and Dendrograptus 

 show that some of the branches consist of a group of 

 tubes of various sizes, somewhat resembling in this respect 

 the recent forms Clathrozoon and Grammaria. 



Dictyonema (= Dictyograptus) is found in the Cam- 

 brian, Ordovician, and Silurian, and has a fan- or funnel- 

 shaped skeleton which consists of numerous radiating 

 branches, placed nearly parallel with one another, and 

 united by transverse fibres. Dendrograptus occurs in the 

 Ordovician, Callograptus in the Arenig, and Ptilograptus 

 ranges from the Arenig to the Ludlow Beds. 



