164 Descriptio7is of Preparations. 



are smooth in both species. It is, of course, necessary to have 

 recourse to the use of the microscope for the verification of these 

 points; and for the detection of the finer spicula, the specimens 

 must be mounted in Canada balsam. Owing to the neglect of 

 this latter method of investigation, the existence of spicula has 

 frequently been overlooked, as, for example, in the case of the 

 Halisarca Difjardinii, which, on account of the supposed absence 

 of these structures, has been elevated to the rank of a distinct 

 ffenus. 



Discussions have been frequently raised, firstly, as to the claim of the 

 Sponges to be considered animal organisms at all ; secondly, as to what 

 is to be considered the unit of their organisms ; and thirdly, as to the 

 rank which the class is to take relatively to other divisions of the animal 

 kingdom. With reference to the first of these questions, which is now 

 all but universally answered in the afiirmative, it may be said that the 

 motile phaenomena noticeable in these creatures, and their great need 

 for, and rapid consumption of oxygen, without which, as supplied by 

 constant additions of fresh- water, the Spongillae may be observed to 

 die and putrefy with very great rapidity, are, from the physiological point 

 of view, very strong evidence for their animal character. The histo- 

 logical evidence however drawn from the detection in them of ciliated 

 as well as non-ciliated epithelium, of cells for the formation of the 

 spicula, and above all of contractile fibre-cells which, though not 

 detected in Spongillae, do exist in the more highly organized Sponges, 

 Cerato-spongiae and Corticatae, is, as Kolliker has remarked, quite con- 

 clusive upon the question. 



It is not possible to give as distinct an answer to the second question, 

 as to whether the Spongiadae are to be considered as colonies made up 

 of uni-cellular but polymorphic organisms, or whether we ought not 

 rather to regard each exhalant osculum as marking out a corresponding 

 zooid, into the constitution of which, as of all higher animals, a mul- 

 titude of polymorphic ' cells ' enter. For, as has been already said, with 

 reference to the Hydroidea, it is not easy always in the lower sub- 

 kingdoms to draw a sharp line of demarcation between what in a higher 

 and indeed sometimes also in a lower organism would be unhesitatingly 

 spoken of as an ' organ, ' and what in nearly allied genera to one which 

 may chance to be under examination would be spoken as a specially 

 modified ' zooid.' It is agreed upon all hands, that, in addition to the 

 nucleated cells, ordinarily but not always, destitute of a cell wall, and 



