SPONGES 91 



The view that the osculum is the sign of the individual, and 

 that a sponge consists of as many persons as there are oscular 

 openings, seems in every way the most natural conception, and it 

 is certainly the conclusion to which embryology leads. Whatever 

 the type of canal system, the metamorphosis of a single larva, or 

 the development of a free bud or gem mule, results in the formation 

 of a small sponge with a single osculum. Not until the osculum 

 is formed can the sponge feed and grow, and perform its usual 

 functions. The osculum represents, therefore, a physiological, as 

 well as a morphological, centre, and thus presents from several points 

 of view the most satisfactory criterion of sponge individuality. 



Although, however, this view is theoretically the most feasible, it, 

 nevertheless, often presents practical difficulties of application in particular 

 instances. We have already seen that, on the one hand, a pseudogaster 

 may be formed by folding up of the body wall so as to enclose a space, 

 primitively external to the sponge, into which the true oscula may open 

 like excurrent canals into a true gastral cavity ; and that, on the other 

 hand, a true gastral cavity may flatten out so that the excurrent canals 

 may come to the surface and simulate oscula. In such cases the physio- 

 logical criteria fail to enable us to recognise the individual, and life- 

 history alone is a guide. Sponges offer great difficulties, in short, to any 

 theory of individuality, and more resemble plants than animals in this 

 respect. The primitively distinct and well-defined individuals become, 

 by increase of the body surface in a vegetative manner, mere growths, 

 zoa impersonalia, in which individuality is more or less completely lost. 



IV. SYSTEMATIC REVIEW OF THE CLASSES AND ORDERS 

 OF SPONGES. 



Since sponges, with very few exceptions, possess a skeleton, 

 composed either of minute spicules of mineral substance, or of fibres 

 of organic nature, it is on the characters of this skeleton that the 

 principal divisions are founded. At the outset one class stands 

 apart from the rest, characterised by a skeleton in which the 

 material is calcareous. Amongst the remainder another group is 

 marked off with almost equal distinctness by the possession of 

 six-rayed spicules of triaxon form. After the separation of these 

 two classes, termed respectively Calcarea and Hexactinellida, there 

 remains a vast assemblage of forms, in which the most divergent 

 types are connected by such a complete and gradual series of inter- 

 mediate forms, that they must be classified together as a single 

 subdivision of the Porifera, equal in value to the other two. To 

 this class the name Demospongiae has been given, and it comprises 

 sponges in which the skeleton may be composed either of siliceous 

 spicules of various types, but never triaxon ; or of fibres of a 

 horny substance, termed spongin, which occurs either pure or in 



