the Sponges to the Corals. 215 



oveiTuled when we come to consider that the animal mass 

 possesses such an amount of irritability and contractility in 

 its dermal membrane that it is enabled to reduce the size of the 

 orifices of its incurrent pores to a mere minimum, or, indeed, 

 to close them altogether. This second condition of affairs 

 (that of the partial closing of the pores) is actually certified 

 by Dr. Bowerbank to exist during the less vigorous action of 

 the ciliary currents in SjwngiUa, Grantiaj and other genera. 

 Now, supposing that this contraction is carried to the utmost, 

 and the incurrent orifices are entirely closed, premising that the 

 ciliary action, which seems to be a fair presumption, is in a 

 constant state of progress, what result should we arrive at ? 

 The terminal osculum would alone remain open, and a sluggish 

 current would probably set in at it, as Hackel and his pupil 

 Miklucho testify to having occasionally witnessed ; and in sup- 

 port of this proposed interpretation, it is a significant fact that 

 Hackel, in recording the phenomenon of a current setting in 

 at the osculum, makes no mention whatever of one setting out 

 at the pores, which, had they been open, must inevitably have 

 taken place*. 



Equal in importance to tlie wide difference which most evi- 

 dently exists between the alimentary and nutritive systems of 

 the two classes in question, is that of the histological structure 

 of the body-mass itself. 



Hackel contends that the tissues of the sponge are as clearly 

 separable into an ectoderm and an endoderm as are those of the 



* A curious demonstration of the involuntary nature of ciliary action 

 was brought before my notice two summers ago. Having for some 

 time kept that interesting and abnormal Polyzoon, Cristatella mncedo, alive 

 in a glass receptacle, it at length, from exhaustion of the supply of food 

 or other causes, died, decayed, and underwent disintegration. One day 

 my attention was drawn to the vessel which had contained it bj' a number 

 of particles of organized matter of various sizes careering about in the 

 water in a most grotesque and extraordinaiy manner — some propelling 

 themselves straight ahead and simply rotating on their axes, others de- 

 scribing circles, parabolic and spiral curves, and a host of other figures, 

 which even a Senior Wrangler would be puzzled to describe. For- 

 getting at the moment what had formerly been placed in the vessel, it 

 first suggested itself that these were some peculiar Infusoria or larval 

 conditions of other higher organisms ; on specimens being examined with 

 the aid of the microscope, however, the fact was revealed that they were 

 nothing more nor less than fragments of the decomposed tentacles of the 

 once translucent CnstateUa, propelled through their ma/y courses by the 

 still active ■vibration of the cilia which clothed them. Now the thorough 

 disintegration of these tentacles must have taken place many days, if not 

 weeks, after the death of the animal; and the motion, moreover, con- 

 tinuing vigorously for a number of days after my first observation of the 

 phenomenon, we have here proof direct of the involuntary nature of ciliary 

 action, if, indeed, we are not justified in describing it as simply a phase of 

 the molecular. 



