SPONGES 



51 



granules ; at first they exhibit lively amoeboid movements, 

 but later pass into a resting stage. The cavity of the 

 mesodenn within which they are situated becomes lined 



Fia. 24. Spermatozoa. a-h, Development of spermatozoa in Sycarulra raph- 

 tmw, highly magnified ; *, mature spermatozoa. After Polejaeff(x792). j, 



A sperm ball in lin-itrdla (u(n(iaris(x500) ; fc, an isolated mature spermatozoon. 

 After Schulze(xSOO). 



by a layer of epithelium, which may not appear, how :ver, 

 till a late stage of segmentation. In Euspongia offia alis 

 the ova occur congregated in groups within the mesod rm, 

 thus presenting an early form of ovary. The spermatozoa, 

 which also develop from wandering amoeboid cells, are 

 minute bodies with an oval or pear-shaped head an-l a 

 long vibratile tail (fig. 24 I). Each amoeboid cell produces 

 a large number of spermatozoa, which occur in spkeiical 

 clusters or sperm-balls. The heads of the spermatozoa, 

 as in the Metazoa, are produced from the nucleus of the 

 mother-cell, the tails from the surrounding protopi sm. 

 The development in detail is upon two plans. In Grantia, 



Family 1. TETRACLAmn/E. With the characters of the sub- 

 order. Examples: Tlicondla, Gray (fig. 21 k); .Dworfcnuw, Bocagc; 

 "SipJumia, rarkinsnn. 



Sub-order 2. RHABDOCRKPIDA. 



The desmas are of various forms, produced by the growth of silica 

 over a uniaxial spicule. 



Family 1. MEGAMORINIP.!;. The desmas are comparatively 

 large. Trurnes, usually dichotri;i/ncs, help to support the ccto- 

 some. Microscleres usually spirastcrs. Examples : Corn/I 7.s-A\, 

 O.S. ; *Hyalotragos, Zittel; Lyidium, O.S. ; *Dorijdcrmin, Zittel. 



Family 2. MICKOMORINID/E. The desmas are comparatively 

 small. Trionies and microscleres are both absent. Examples : 

 Azorica, Crtr. ; *TcrrutUiM, Zittel. 



Sub-order 3. ANOMOCLADINA. 



Desmas with a massive nucleated centrum, from which a variable 

 number of arms (zS) extend radiately (see fig. 12/). Examples: 

 Vctulina, 0. S. ; *Astylosponrjia, Roemer. 



Reproduction and Embryology. 

 Fresh individuals arise by asexual gemmation, both 



external and internal, by fission, and by true sexual repro- 

 duction. 



Asexual Fission is probably one of the processes by which com- 

 multipli- pound sponges are produced from simple individuals, 

 cation. Artificial fission has been practised with success in the 



cultivation of commercial sponges for the market. Ex- 

 ternal gemmation has been observed in Tlienna, Tethya, 



Polymastia, and Oscarella. A mass of indifferent sponge- 

 cells accumulates at some point beneath the skin, bulges 



out, drops off, and gives rise to a new individual. Internal 



gemmation, which results in the formation of a statoblast, 



is only known to occur in the freshwater Spongillidx, 



The statoblasts consist of a mass of yolk -bearing 



mesoderm cells, invested by a capsule, which in 



Ephydatia fluviatilis is composed of an inner 



cuticle of spongin separated from a similar outer 



layer by an intermediate zone of amphidisks and 



interspersed protoplasmic cells. On one side of 



the capsule is a hilum which leads into the interior. 

 Their development has recently been studied by Giitte, 



with results that confirm the conclusions of Carter (j) 



and Lieberkiihn (fj). The process commences with an 



accumulation of amoeboid cells within the mesoderm to 



form a globular cluster ; yolk grannies develop within 



them, especially in those that lie nearer the centre. The 



external cells give rise to the investing capsule ; some 



resemble sponginblasts and secrete the inner and outer 



horny cuticle ; others give rise to the amphidisks and 



interspersed cells of the middle layer. Under favourable 



conditions the interior cells creep out through the pore 



of the capsule, and form a spreading heap, which by 



subsequent differentiation gives rise to a youug Spongilla. 



Since the freshwater sponges can only be regarded as 



modified descendants of ancient marine species (prob- 

 ably of the family Ifttlichondridse), we may consider the 



internal gemmules, like the similar statoblasts of the 



freshwater Poli/:oa, as special adaptations to a changed 



mode of life. They appear primarily to serve a protective 



purpose, ensuring the persistence of the race, since they 



only appear in extreme climates on the approach of 



drought, and in cold ones on the approach of winter. 



As a secondary function they serve for the dispersal of 



the species ; some are light enough to float down a 



stream, but not too far, so that there is no danger of 



their being carried to sea ; others, which are character- 

 ized by large air-chambers, are possibly distributed by 



the wind. 



Sexual Both sexual elements may be formed in the 

 repro- same individual, e.ff., Oscarella lobularis, Grantia 

 on. raphanus, and many others ; but even in herm- 

 aphrodites one or other element usually occurs to 



excess in different individuals, SO that Some are F[G 25 _ Development of a calcareous sponge (Symndra raphanw). a, ovum ; 6, c, ovum seg- 

 predominantly male and Others predominantly mented,-&, as seen from above, c lateral view; rf blastosphere; f, ampl.il.lastula; /corn- 

 if: 7 J meneement nf the invagination of the flagellated cells of the amphiblastula ; </, gastrula 



female, rolejaeff found Only one such male form attached by its oval face; h,j, young sponge (Ascon stagi-), ft, lateral vi-, j, as seen from 



to 100 female forms in Grantia raphanus. In above - After Schuize. 



Other sponges Beniera fertilis, Euspon,,i<i otfr.-itialisihe raphanus (/j) the nucleus of the mother-cell divides into two 



,? . mi i (fi" 24 b) one of the resulting daughter nuclei undergoes no 



sexes are distinct. The ova develop from archaeocytes or ^^ ^ but with a gmall ^^ of ri heral protop i asm 



wandering amoeboid cells, which increase in size and ac- forms a "cover-cell" to the other or primitive sperm nucleus and its 

 quire a store of reserve nourishment in the form of yolk associated protoplasm. The sperm nucleus repeatedly divides, with- 



