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HARDWICKE'S SCIENCE-GOSSIP. 



of the sarcode particles take on the structure of ova, 

 each of these being provided with a yolk and ger- 

 minal vesicle. Other particles have their contents 

 converted into spermatozoa, and by contact of these 

 with the ova, a free-swimming embryo is produced, 

 which, after a time, becomes attached and developes 

 into the sponge. 



The phenomena of asexual reproduction have been 

 worked out in the freshwater sponge (Spongilla), and 

 are very interesting and curious. The more central 

 portion of the sponge gets occupied with small round 

 bodies which have been called "gemmules." Each 

 of these is made up of a leathery integument, and is 

 provided with an aperture at one end. 



The gemmule is invested with a layer of spicules 

 of a peculiar shape, resembling two wheels united 

 by an axle. Each wheel is provided with teeth. 

 These spicules are so arranged that one "wheel" is 

 in contact with the surface of the gemmule, the other 

 being free. The interior of the gemmule is occupied 

 by a number of cells, the central ones containing 

 each a germ, and by the escape of these cells through 

 the aperture in the gemmule, the spongilla is pro- 

 pagated — for each one of these germs, on reaching a 

 suitable habitat, developes into a spongilla. 



It may be mentioned that this asexual mode of 

 reproduction in spongilla only takes place in the 

 winter-time. Before leaving the subject of the develop- 

 ment of sponges, it may be well to observe that 

 recent researches render it probable that the sponges 

 will have to be taken from their present position in the 

 animal kingdom, and be more closely allied to the 

 sea anemones and fresh and salt water polypes, certain 

 discoveries in relation to the phenomena of their 

 development making this change of position abso- 

 lutely necessary by approximating them more closely 

 to the hydrozoa and actinozoa which constitute the 

 sub-kingdom Cselenterata. Until recently the sponges 

 have been regarded as forming one of the groups of 

 the sub-kingdom Protozoa, a sub-kingdom which 

 includes a large number of animals low down in the 

 scale of organisation. Two specimens of this sub- 

 kingdom may be mentioned (in addition to the 

 sponges), Amoeba and Vorticella. These animals are 

 good representatives of the limits of the Protozoa in 

 each direction, viz., of greatest simplicity and of 

 greatest complexity, for the amoeba is one of the 

 simplest of all known animals, being strictly com- 

 parable to any one of the sponge, or sarcoid, par- 

 ticles already mentioned. In this creature no part of 

 the body is differentiated, that is to say, is set aside 

 for any one function. There is no division of labour. 

 But the bodily functions of absorption, assimilation, 

 digestion, and motion, are performed by any portion 

 of the body indifferently. Such an animal is indeed 

 of a simple organisation, and much resembles a 

 portion of animated jelly. On the other hand the 

 vorticella is, comparatively speaking, of a complex 

 organisation, possessing considerable differentiation 



of tissues and organs, and being provided with a 

 mouth and short digestive canal. The beautiful 

 appearance presented by the graceful vorticella, with 

 its spiral stalk passing through different stages of 

 compression and extension, must be well known to 

 all microscopical observers. 



The affinities of sponges have long been disputed 

 points with comparative anatomists ; some endeavour- 

 ing to show that the sponges are closely allied to the 

 Infusoria. Others, again, as has been mentioned 

 above, from recent observations think, and with 

 much probability, that the sponges would be removed 

 to the Ccelenterata. It is not, however, the purpose 

 of this paper to enter into the question, which to be 

 adequately understood, requires a considerable 

 acquaintance with the formal facts of comparative 

 anatomy and of embryology. 



Having thus given in brief outline some of the 

 leading facts concerning the structure and physiology 

 of the sponges, it will be necessary to say a few words 

 on the distribution, both in space and time, of this 

 interesting group of animals. As regards distribution 

 in space, the sponges are almost entirely confined to 

 salt water, the genus Spongilla comprising the only 

 fresh-water sponges. They occur almost universally ; 

 but those whose structure especially fits them for 

 domestic use are obtained chiefly from the islands of 

 the Grecian Archipelago, and from the Bahamas. It 

 will, of course, be perceived that the species provided 

 with a horny skeleton, and comparatively devoid of 

 spicules, are those which are most valuable as a com- 

 mercial article. Other things being equal, the utility 

 of the sponge will vary inversely as the density of 

 the skeleton and the number and hardness of the 

 spicules. Sponges "occur chiefly between high and 

 low water marks, and are found in most luxuriance 

 and abundance in tropical seas. It has been shown 

 that the siliceous sponges mostly occur at great depths 

 in the ocean. There is one genus which is worthy of 

 notice from its habit of boring cavities in shells — this 

 genus is Cliona. Fossil-shells from the Silurian are 

 found bored in this manner, excavated doubtless by a 

 boring-sponge. Sponges are very widely distributed 

 in time. Indeed, from the Palaeozoic strata upwards, 

 sponges occur in many formations. The maximum of 

 abundance is reached in the chalk. And it is worthy 

 of notice that the flints of the upper chalk are formed 

 round sponges, forming the nucleus, as it were, or 

 centre of deposition. On sections being made of 

 certain flints, minute spherical bodies, provided with 

 spines, have been discovered, and to these the name 

 Xanthidia has been applied. Some observers regard 

 these as the gemmules of sponges ; but on this point 

 there is considerable difference of opinion, as there is 

 some reason for regarding them as members of the 

 vegetable kingdom, namely, the spore cases of 

 Desmids. (?) It is obvious that the only trace left by 

 the horny sponges can be owing to their spicules, and 

 consequently remains of these sponges are rare. 



