HALICHONDRIA. 



127 



visible, and the central tube very rarely so comparatively. Of 

 the forms, No. 1 is about the most common and 4 the most 

 uncommon ; the others are of frequent occurrence." 



Fig. 13 

 2 3 4 



QOi^) («) 



When this zoophyte inhabits and fills up the worm-like holes 

 in old oyster-shells its form is modified and then " depends on 

 that of the cavities w hich it fills ; it insinuates itself into their 

 minutest ramifications, and adheres so closely to their smooth 

 parietes, that it cannot be separated without tearing." The 

 part which projects beyond the orifice of the hole is papillary, 

 about a line in height and about the same in breadth, of a yel- 

 lowish colour, tubular, and cither closed or widely open at the 

 apex. In texture, and in the mode in which the spicula are dis- 

 posed amid the parenchyma, this production resembles Hali- 

 chondria panicea or H. incrustans. It occupies " the perfo- 

 rated shells of the living oyster, as well as the detached valves 

 of the dead animal ; but, in the living oyster, as the perfora- 

 tions are only seen on the outside, and never pass through the 

 innermost layer, there is always a thin layer of shell between 

 the yellow substance and the living animals. On the death of 



