SPONGES 



53 



is a freshwater form, anomalies in its development (which remind 

 us of those in the development of the freshwater Sydra) might 

 almost be expected. 

 Homo- Probably in no other single group is the doctrine of 

 plaay. homoplasy enunciated by Lankester more tellingly illus- 

 trated than in the sponges. The independent develop- 

 ment of similar types of canal system in different groups, 

 sometimes within the limits of a single family, is a remark- 

 able fact. In the following table the sign x shows inde- 

 pendent evolution of similar types of canal system in 

 different groups : — 



Physio- 

 logy. 



In the gross anatomy of the canal system similar homo- 

 plasy obtains; thus, to cite one case amongst many, a 

 peculiar type of canal system characteristic of Siphonia 

 (Lithistid) occurs also in^7reptoca(Hexactinellid),»S'cAmirf<ia 

 (Monaxonid), and other apparently unrelated genera. The 

 development of a cortex has likewise taken place inde- 

 pendently, but on parallel lines, in the Syconidx, Leu- 

 conidss, Monaxona, Tetillidse, and Stellettidx. Calcareous 

 and silicious spicules have evidently an independent his- 

 tory, and yet all the chief forms of the former are repeated 

 in the latter. Quite as remarkable is the similarity of 

 the independently evolved horny spicules of Darmnella 

 aurea to th-e quadri- and sex-radiate silicious spicules. We 

 have now sufficient knowledge of the morphology and evolu- 

 tion of the sponge to furnish the physicist with data for an 

 explanation of the skeleton, at least in its main outlines. 

 The obvious conclusion from this is that variation does not 

 depend upon accident, but on the operation of physical 

 laws as mechanical in their action here as in the mineral 

 world. Another important consequence follows : if homo- 

 plasy — i.e., the independent evolution of similar structures 

 — is of such certain and quite common occurrence in the 

 case of the sponges, it is also to be looked for in other 

 groups, and polyphylitic origin, so far from being improb- 

 able, is as likely an occurrence as monophylitic origin. 



Physiology and JEtiolbgy. 



Under the head of "physiology" we have almost a 

 blank. At present we do not even know what cells of the 

 sponge are primarily concerned in the ingestion of food. 

 If a living sponge, such as SpongUla, be fed with carmine 

 for a few minutes, then immersed in dilute osmic acid, and 

 examined in thin sections, its flagellated chambers are 

 found to be all marked out as red circular patches, and a 

 closer investigation shows that the choanocytes, and they 

 alone, have ingested the carmine. In this way we con- 

 firm the earlier observations of Carter made by teasing 

 carminerfed sponges. This might be thought to decide 

 the question; but, though it effectually disposes of Pole- 

 jaeff's argument that the choanocytes do not ingest nutri- 

 ment because mechanical disadvantages (conceived a priori) 

 make it impossible, it has not proved a final solution. Von 

 Lendenfeld, by feeding sponges such as AplysUla with 

 carmine for a longer interval — a quarter of an hour — finds 

 that amoeboid cells crowd about the sides and particularly 

 the floor of the subdermal cavities, and are soon loaded 

 with carmine granules ; after a time they wander away to 

 the flagellated chambers and there cast out into the ex- 

 current canals the carmine they have absorbed, apparently 



in an altered state. On the other hand, the choanocytes, 

 though they at first absorb the carmine, soon thrust it out, 

 apparently in an unaltered state. Hence Von Lendenfeld 

 concludes that it is the epithelium of the subdermal cavities 

 which is charged with the function of ingestion, and that 

 the amoeboid cells subsequently digest and distribute it, 

 and finally cast out the worthless residues. There may be 

 much truth in this view, but it requires to be supported 

 by further evidence. (1) Sufficient proof is not adduced 

 to show that the carmine granules expelled from the amoe- 

 boid cells are really more decomposed than those rejected 

 by the choanocytes. (2) There is at present no proof that 

 carmine is a food, or that if it is sponges will readily feed 

 upon it. In either case one would expect the amoeboid 

 cells to play the part which they perform in other organisms 

 and to remove as soon as possible useless or irritant matter 

 from the surface which it encumbers ; at the same time 

 the choanocytes, not having found the food to their liking, 

 would naturally eject it. (3) If the choanocytes do not 

 ingest food, how does the Ascon feed, since in this sponge 

 all the pinnacocytes are external ? It is, however, a very 

 noticeable fact that, as the organization of a sponge 

 increases in complexity, the choanocytal layers become 

 reduced in volume relative to the whole bulk of the 

 individual; and it is quite possible that as histological 

 differentiation proceeds it may be accompanied by physio- 

 logical differentiation which relieves the choanocytes to 

 some extent of the ingestive part of their labours. 



The origin of the sponges is to be sought for among jEtio- 

 the choanoflagellate Infusoria ; and Savile Kent has de- logy- 

 scribed a colonial form of this group which is suggestively 

 similar to a sponge. Its differences, however, are as 

 marked as its resemblances, and have been sufficiently 

 pointed out by Schulze {23). Kent has called this form 

 Protospongia, a name already made use of, and fortunately, 

 as the organism is not in any sense a true sponge ; the 

 present writer proposes, therefore, to call it Savillia, in 

 honour of its discoverer. It consists of choanoflagellate 

 Infusoria (see Pkotozoa, vol. xix. p. 858, fig. XXI., 15), 

 half projecting from and half embedded in a structureless 

 jelly or blastema, within which other cells of an amoeboid 

 character and reproductive function are immersed. Pro- 

 fessor Haddon arrives at the generalization that conjuga- 

 tion amongst the Protozoa always takes place between 

 individuals of the same order : flagellate cells conjugate 

 with flagellate, amoeboid with amoeboid, but never with 

 flagellate ; while in true sexual reproduction the conjuga- 

 tion occurs between two individual cells in different stages 

 of their life cycle : a flagellate cell conjugates with a resting 

 amoebpid cell. Now Savillia would appear to be extremely 

 near such a true sexual process, since the simultaneous 

 coexistence of cells in two different stages of life and 

 within easy reach of each other — a necessary preliminary, 

 one would think, to the union — has already been brought 

 about. That coalescence between two different histological 

 elements should result in products similarly histologically 

 differentiated (compare amphiblastula stage of Calcarea) 

 has in it a certain fltness, which, however, has still to be 

 explained. The mode by which an organism like Savillia 

 might become transformed into an Ascon cannot be sug- 

 gestively outlined with any satisfactory results till our 

 knowledge of the embryology of sponges is more advanced. 

 The minute characters of the flagellate cells of the amphi- 

 blastula and other sponge larvae are still a subject for 

 research. They often possess a neck or coUum ; but the 

 existence of a frill or collar is disputed. Kent asserts 

 that it is present in several embryos which he figures ; 

 and Barrois makes the same assertion in respect to the 

 larva of Oscarella, and illustrates his description with a 

 figure. On the other hand, Schulze and Marshall both 



