89 



agency of life. The Hippothoa, and all the encrusting species 

 grow very similarly. From one, and sometimes from three 

 points in Hippothoa, a gelatinous transparent looking sub- 

 stance is effused of the length, breadth, and form of the future 

 cell, and in this the lime is soon deposited in the form of a 

 perfect cell. The Tubuiiporidce, from the earliest periods in 

 which they can be observed have open mouths ; in a very 

 early state, the polype may be said to exist without a tube, 

 since the calcareous portion forms but a base for the cell, but 

 as the polype elongates the tube also increases in length. 



Whatever opinion may be entertained regarding the poly- 

 pidoms of the foregoing orders, those now under consideration 

 are generally allowed to have an organic connection with the 

 polype ; but the evidence does not appear to be more con- 

 clusive here than in the other orders. In the Sea Mats 

 ( F lustra J and the kindred genera, the manner in which their 

 organic nature can be ascertained, can be seen to the best 

 advantage, and to these a few observations will be directed. 



The encrusting species vary a great deal in shape, which 

 depends in a great measure on their different modes of growth. 

 The Hippothoa has three points only from which growth 

 takes place : the terminal, which is the most fruitful, and 

 one on each side of the cell. At these points a semi-fluid 

 transparent substance is effused, of the form of the cell; and 

 hence in young specimens the cells are connected together 

 like loosely strung beads. In old specimens, where each 

 cell has had time to extend its growth from the three points, 

 it greatly resembles an irregular Flustra. In the Flustra 

 membranacea, an exceedingly delicate and guaze-like species, 

 there appears to be only one spot for extension, which is not 

 a point, for it extends over all the distal surface of the cells ; 

 but this gives rise to another which may be called accidental, 

 since it arises from a peculiarity in the cell. The terminal 

 or longitudinal increase is produced by an effusion of a seniN 

 solid gelatinous substance, which is called a pulp, and in 

 which the cells are formed. This pulp extends to a certain 

 distance, differing in different specimens, which marks the 

 extent of the growth for one time. In a very short time 

 after the pulp is effused, faint white or milky looking streaks 

 are observed to traverse the pulp in a longitudinal direction 

 so far as the pulp extends. These lines form the lateral 

 boundaries of the future cells. At first the lines are faint, 

 but they soon become distinct and well defined. Faint 

 milky lines are then observed to pass transversely and thus 

 divide the space into cpaadrangular compartments : these are 

 much stouter than those first formed. The form of the cell 

 now being completed, no further alteration takes place, but 

 the further condensation of the sides and the formation of the 



