12 CLASSIFICATION OF THE ANIMAL KINGDOM. 



while others present an equally close resemblance to Amcebce; and 

 others have more or less completely coalesced to form a gelatinous or 

 mucilaginous common matrix or " cytoblastema." This protoplasmic 

 aggregate may or may not be supported by a skeleton of diverse composi- 

 tion and structure ; but it is always so disposed as to be traversed by a 

 series of canals, which convey water in and out of the organism, and are 

 connected with respiration and the procuring of food. These canals com- 

 mence on the surface by numerous small "inhalant" apertures or 

 " pores," which admit the external water, and they ramify through the 

 substance of the Sponge. They ultimately open on the surface by a series 

 of " exhalant " canals, which converge to a common aperture of large 

 size the so-called " osculum," which serves as an outlet for the water- 

 currents. The entire system of water-canals may be lined with flagellate 

 sponge-particles, similar in structure to Flagellate Infusoria ; or they are, 

 more commonly, dilated at intervals into globular chambers, which are 

 lined by these flagellate sarcoids (fig. 8), the vibrations of the flagella of 

 these serving to keep up a circulation of water through the body of the 

 Sponge. Lastly, a Sponge may consist of one excretory opening or " oscu- 

 lum," together with the " pores " belonging to this ; or it may consist of 

 a larger or smaller number of such oscula, each with its proper comple- 

 ment of " pores." 



Until within the last few years, Sponges have been generally regarded 

 by naturalists as belonging to the Protozoa, and as either referable to the 

 Rhizopoda, or as constituting a separate division of Protozoa. Recently, 

 however, the view has been put forward by Professor Hseckel, and has 

 been largely accepted by zoologists, that the Sponges are properly Metazoa, 

 and that they are truly allied to the Corals, and therefore properly refer- 

 able to the Ccelenterata. As a modification of this view, the Sponges are 

 regarded as constituting a group of Metazoa intermediate between the 

 Coelenterates and the Protozoa. If Hseckel's view as to the affinities of 

 the Sponges be received, it is necessary to accept the view which this dis- 

 tinguished writer advocates with regard to the development of the Sponges, 

 namely, that the ovum of the Spongida undergoes a regular process of 

 " segmentation," consequent on fecundation by a spermatozoon, and that 

 it becomes converted into an embryo (" gastrula ") composed of an outer 

 and inner cellular layer, enclosing a central cavity. On the other hand, 

 a large amount of evidence has been brought forward by various observers, 

 and notably by Mr Saville Kent, which would go to show that true sexual 

 reproduction, by means of proper " ova " and " spermatozoa," is of very 

 doubtful occurrence among the Sponges ; and that it is very questionable, 

 therefore, if there is truly any such phenomenon in their development as 

 the " segmentation " of an ovular cell. The supposed two-layered " gas- 

 trula " of the Sponges would rather appear to be really an asexually pro- 

 duced " swarin-gemmule," composed partly or wholly of flagellate zooids 

 or monads, entirely similar in their structure to the Flagellate Infusoria, 

 and resulting from the segmentation of a single " sponge-particle " of the 



