Sarcostyles of the Plumularida?. 121 



localities, however, they did not move from place to place, 

 but. nevertheless sent forth numerous pseudopodia and ex- 

 hibited amoeboid change of form. 



This demonstration of an axial cavity in the sarcostyle is 

 of considerable interest, in view of the fact that it furnishes 

 the last and much-desired link in the evidence needed to 

 demonstrate the homology of the sarcostyle. It can no longer 

 be doubted, it seems to me, that the sarcostyle is the homo- 

 logue of the hydranth — that it is, in fact, a true " person" of 

 the hydroid colony, being composed of ectoderm, " Stutz- 

 lamelle," endoderm, and body-cavity. It lacks only tentacles 

 to make it a hydranth ; but we know that certain hydroids, 

 e.g. Protohydra, have undoubted hydranths without tentacles. 



Curiously enough, one of the earliest observers of nemato- 

 phores published in 1863 a figure of a sarcostyle which was 

 represented as having a body- cavity. The author referred to 

 is Semper, and the figure is found in the 'Zeitschrift fur 

 wiss. Zoologie,' Bd. xiii. pi. xxxviii. fig. 4 a. 



The conclusion that sarcostyles are morphological persons 

 of the colony is borne out by almost every known fact con- 

 cerning them. Embryological investigation shows that they 

 are formed in almost exactly the same manner as the 

 hydranths, and that they make their appearance as early as 

 the latter, and often earlier. It is possible, moreover, to 

 point out a series of forms leading from the so-called " fighting 

 zooids " of Hydractinia to the typical nematophores of the 

 Plumularidaj. In the genus Ophiodes we find organs or 

 persons almost exactly intermediate between the Hydractinia 

 and true sarcostyles. Prof. Baldwin Spencer has lately 

 described a new family of Hydroida, called the Hydrocera- 

 tenidaa, evidently closely allied to the Plumularidse, with 

 numerous fighting persons which are histologically almost 

 identical with true nematophores ; the extreme extensibility, 

 however, of the latter has not as yet been observed in the 

 former. 



There appears also to be a curious cross-relation between 

 the dactylozooids of the Millipora and the sarcostyles, if such 

 they be, of the Hydroceratenidaj. 



Among the many perplexing questions in this connexion is 

 the one raised by Professor Allman, who very strongly urges 

 the relationship between the nematophores and the denticles 

 of the graptolites. His argument would lead to a belief that 

 the ancestors of the Plumularida? may be the graptolites; 

 that the nematophores of the former are the homologues of the 

 denticles of the latter ; that we have in the sarcostyle the 



