54 



SPONGES 



deny its existence, and the former attributes Kent's 

 observations to error. One constant character they do 

 possess : they are provided with rlagella at some stage of 

 their existence, but never with cilia. Ciliated cells, in- 

 deed, are unknown amongst the sponges, and, when pinna- 

 cpcytes exceptionally acquire vibratile filaments, as in 

 Oscarella and other sponges, these are invariably flagella, 

 never cilia. An Ascon stage having been reached at some 

 point in the history of the sponges, the Sycon tubes and 

 Khagon chambers would arise from it by the active pro- 

 liferation of choanocytes about regularly distributed centres, 

 possibly as a result of generous feeding. Vosmaer recog- 

 nized as the physiological cause of Sycon an extension of 

 the choanocytal layer. Polejaeff, relying on Von Lenden- 

 fekl's experiments, which seem to prove that it is the 

 pinnacocytes and not the choanocytes which are concerned 

 in the ingestion of nutriment, argues that, as in Sycon 

 the pinnacocytal layer is increased relatively to the choano- 

 4 cytal, we have in this a' true explanation of the transition. 

 The existence of Homoderma, Lfcl., however, shows that 

 in the\ first stage there was not a replacement of choano- 

 cytes by pinnacocytes, but that this was a secondary 

 change, following the development of radial tubes, and 

 therefore cannot be relied upon to explain them. The 

 radial tubes having been formed by a proliferation of 

 choanocytal cells, the reduction of those. lining the para- 

 gastric cavity to pinnacocytes would follow in consequence 

 qf the poisonous character of the water delivered from the 

 radial tubes to the central cavity, since this water not 

 only parts with its dissolved oxygen to the choanocytes 

 it first encounters, but receives from them in exchange 

 urea, carbonic acid, and fascal residues. The development 

 of subdermal cavities is explicable on Von Lendenfeld's 

 hypothesis. 



Distribution. 



Distribu- Our knowledge of this subject is at present but frag- 

 tiou m rnentary ; we await fuller information in the remaining 

 reports on the sponges obtained by the " Challenger." The 

 sponges are widely distributed through existing seas, and 

 freshwater forms are found in the rivers and lakes of all 

 continents except Australia, and in numerous islands, in- 

 cluding New Zealand. Many genera and several species 

 are cosmopolitan, and so are most orders. 



As instances of the same species occurring in widely remote 

 localities we take the following from Polejaeff : Si/con arclicum is 

 found at the Bermudas and in the Philippine Islands, as also are 

 Leuconia multiformis a-nd Leucilla uter ; Sycon raphaniis occurs at 

 Tristan da Cunha and. the Philippines ; Hctcropcgma nodus-gordii 

 and Leuconia dura at the Bermudas and Torres Straits. We do not 

 know, however, whether these species are isolated in their distribu- 

 tion or connected by intermediate localities. Of the Calcarea about 

 eighty-one species have been obtained from the Atlantic, twenty- 

 two from the Pacific, and twenty-two from the Indian Ocean ; but 

 these numbers no doubt depend largely on the extent to which the 

 several oceans have been investigated, for the largest number of 

 species has been found in the ocean nearest home. Schulze states 

 that the ffexattinellida brought home by the "Challenger" were 

 obtained at seventeen Atlantic stations, twenty-seven Pacific, and 

 nineteen in the South Seas.' In the last the number of species 

 was greatest, in the Atlantic least. They flourish best on a 

 bottom of diatomaceous mud. The Calcarea and Ccmtosa are 

 most abundant in shallow water and down to 40 fathoms, but 

 they descend to from 400 to 450 fathoms. The ff.'.i'.i<iini'!!ida are 

 most numerous over continental depths, i.e., 100 to 200 fathoms; 

 but they extend downwards to over 2500 fathoms and upwards 

 into shallow water (10 to 20 fathoms). The Li'lltintii/n are not such 

 deep-water forms as the HcxactincUida, being most numerous from 

 10 to 150 fathoms. Only one or two species have been dredged 

 from depths greater than 400 fathoms, and none from 1000 fathoms. 

 The Clwristida range from shallow water to abyssal depths. A 

 characteristic deep-sea Choristid genus is Tlicnm, Gray (= U'yrill? 

 Thompsonia, Wright ; Dorvillia, Kent). This is most frequently 

 dredged from depths of from 1000 to 2000 fathoms ; but it extends 

 to 2700 fathoms on the one hand and to 100 on the other, 

 in time. Until about 1876 one of the chief obstacles to the inter- 



space ; 



^relation of fossil sponges arose from a singular mineral 

 replacement which most of them have undergone, leading 

 to the substitution of calcite for the silica of which their 

 skeletons were originally composed. This change was de- 

 monstrated by Zittel (jj) and Sollas (24), and, though it 

 was at first pronounced impossible, owing to objections 

 founded on the chemical nature of silica, it has since be- 

 come generally recognized. These observers also showed 

 that the fossil sponges do not belong to extinct types, but 

 are assignable to existing orders. Zittel in addition sub- 

 jected large collections to a careful analysis and marshalled 

 them into order with remarkable success. Since then 

 several palaeontologists have worked at the subject, Pocta, 

 Dunikowski, and Hinde (/), who has published a Cata- 

 logue which is much more than a catalogue of the 

 sponges preserved in the British Museum. The result of 

 their labours is in general terms as follows. Fossil sponges 

 are chiefly such as from the coarseness or consistency of 

 their skeletons would be capable of preservation in a miner- 

 alized state. Thus the majority are Hexactinellida, chiefly 

 Dk-tyonina Tetractinellida, chiefly Lithistida ; and Cal- 

 carea, chiefly Leuconaria. Monaxonid sponges rarely occur; 

 the most ancient is Climacospongia, Hinde, found in Sil- 

 urian rocks. A very common Halichondroid sponge of this 

 group (Pharetrospongia strakani, Soil.) occurs in the Cam- 

 bridge greensand; it owes its preservation to the collection 

 of its small oxeate spicules into dense fibres. The Clwristida, 

 though not so common as the Lithistids, are commoner 

 than the Monaxonids, particularly in Mesozoic strata. 



The distribution of fossil sponges in the stratified systems may 

 be summarized as follows. CALCAREA. Homoccela, none. Hetcro- 

 ccela, a Syconid, in the Jurassic system. Numerous Leuconaria 

 from the Devonian upwards. MYXOSPONGM!. None; not fitted 

 for preservation. HEXACTINELLIDA. Lyssacina, from the Lower 

 Cambrian upwards. Dictyonina, commencing in the Silurian ; most 

 numerous in the Mesozoic group ; still existing. MOXAXONIDA. 

 Moiiaxona, from the Silurian upwards. Ccratostt, none ; few are 

 fitted for preservation. TETRACTINELLIDA. Choristida, from the 

 Carboniferous upwards ; most numerous in the Cretaceous system. 

 Lithistida, from the Silurian upwards ; most numerous in the 

 Mesozoic group. In ancient times the Hexactinellids and Lithistids 

 seem not to have been so comparatively uncommon in shallow 

 water as they are at the present day. Thus, in the Lower Jurassic 

 strata of the south-west of England we find Dictyoniue Hexactinel- 

 lids, Lithistids, and Leuconarian Calcarea associated together in a 

 shelly breccia and in company with littoral shells, such as Patella 

 and Trochus. Several Palfeozoic Hexactinellids actually occur in a 

 fine-grained sandstone. Of the Chalk, which is the great mine of 

 fossil sponges, we must speak with caution, owing to the insufficient 

 evidence as to the depth at which it was deposited. 



As shown by Protospongia, the phylum of the sponges was in 

 existence in very early Cambrian times, and probably much earlier. 

 Before the end of the Silurian period its main branches had spread 

 themselves out, and, developing fresh shoots since then, they have 

 extended to the present day. Of the offshoots none of higher value 

 than families are known to have become extinct, and of these 

 decayed branches there are very few. The existence in modern 

 seas of the Asconidse, which must surely have branched off very 

 near the base of the stem, is another curious instance of the per- 

 sistence of simple types, which would thus appear not to be so vastly 

 worse off in the struggle for existence than their more highly 

 organized descendants. 



Biblloaraphti. A fairly complete list, "f works on sponges published before 

 1SS2 will be found in Vosinaer's article "Fcirifcni"," in Bronn's Klassen und 

 Onlnnngcn, vol. ii. D'Arcy Thompson's Catalogue of Papers on Piotozoa and 

 Ccdenterata, a still more complete list, extends to 1884. 



The following is a list of works, including those referred to in the preceding 

 pages : (/) C. Barrois, Embryologtt tt. qn,l,i,,rs Sponges d. I. Miim-h?, I':iris, is7ii. 

 (?) Bowei-tank, .-I Monograph nf n,ilisli Spongiartre, vols. i.-iv., 1SU4-S2 (vol. 

 iv. is posthumous, edited by Dr Norman), (j) Carter, a series of papers in the 

 A a a. ii ail .W,i:i. KnI. His/., from 1847 to the present time (18ST). (./) J. Clark, On 

 tlif Bponeiffi ciliatn> a* Infusoria flngcllata, ISii.j. (5) Grant, Eilin. Pliil. Jmn-n., 

 1S2.1. (6) Haeckel, jlfontwmplu il. Kiillcsi-lni'iimme, 1S71. (-) Ilindt, .4 I'ulii- 



logutaftheSpongeainthe British Museum, 1883. (flld., "On Hie Li-iTjjui-nliHil.-it." 

 in Quart. Journ. Gcol. Snr., xl. 705, 1884. (0) Keller, "Stndien ii. Organisation 

 u. Entwickelung d. Chalinecn," in Ztschr. f. wiss. Zoo}., xxxiii., 1S79. (10) Kent, 

 "Notes on the Embryology of the Sponges," in Ann. inul Mart. Nat. Hat., 

 1S7S, ii. 139. (//) Von Lendenfeld, "On Aptmimtda," in Ztschr. f. wiss. Zool., 

 xxxviii. (/?)Id., "A Monograph of Australian Sponges," in Proc. Linn. Snc.,N.S. 

 JlWra, vols. ix., x. (other papers by Von Lendenfeld will lie found under this 



and Spermatogenesis in Sycandra raphanus," in Site. -Her. Acad. wiss. Zool., 



