SPONGES 89 



Sponges protect their bodies, and especially their apertures, against 

 the attacks of intruders or enemies by fringes and palisades of spicules, 

 and also by excretion of poisonous ferments from the surface of the body 

 which have a strongly oxidising action (Spr t wjilla, Loisel). It is perhaps 

 to this that the smell of sponges is due. 



As competitors sponges are very dangerous enemies to animals which 

 feed in a similar manner, such as Lamellibranchs, since they grow over 

 their shells and starve them by forestalling their supply of food. In 

 oyster culture a method of preventing this is to grow the oysters on 

 frames, which are occasionally pulled up and exposed during a shower of 

 rain. The fresh water kills the sponges, but the oysters close their shells 

 and are unscathed. 



No adult sponge is known to be sensitive to light, but this property is 

 often exhibited by the larvae in a marked degree. The larvae of Ascons 

 are positively heliotropic when newly hatched, and swim at the surface. 

 They then become indifferent to light for a time, which is followed by a 

 third period, during which they are negatively heliotropic and swim at 

 the bottom, previously to fixing themselves. The sensitiveness appears to 

 reside in certain highly refringent granules in the ciliated cells, which in 

 the amphiblastulae are aggregated at the inner ends. In many siliceous 

 larvae there is a patch of pigment at the hinder end, which the larva 

 tends to turn towards the light, with the result that the larva as a whole 

 moves towards the dark. 



Individuality. The discussion of the morphology and physiology 

 of sponges may well be terminated by attempting an answer to the 

 question : What constitutes the individual in a sponge ? The 

 most divergent views have been expressed on this point. 



The opinions that have been put forward with regard to the 

 constitution of the sponge body by different authors depend, of 

 course, largely upon the views held by them as to the affinities of 

 the group (see below, p. 158). While most of the older writers 

 regarded the cell as the unit of individuality in a sponge, more 

 recent scientific opinion has sought to identify the sponge person 

 with some form of cell aggregate namely, either with the flagellated 

 chamber, or with so much of the canal system as is centred round 

 a single osculum. 



The older observers regarded the sponges as Protozoan colonies, con- 

 sisting of an aggregate of amoebae or Infusoria (Perty, Dujardin, Lieber- 

 ku'hn, Carter, and Savile-Kent), until the discovery by James-Clark (1867) 

 of the collar cells, and their resemblance to Choanoflagellata, led him and 

 others to regard them as a colony of Choanoflagellata. This view was 

 taken up by Savile-Kent and Carter, the latter terming the collar cell the 

 " spongozoon." At the present day these views and the controversies to 

 which they gave rise have little more than a historical interest. 



The view that the sponge person was represented by the flagellated 

 chamber, held at one time by Carter, has its chief advocate in Haeckel, 

 and is based upon a theoretical interpretation of the origin of the canal 



